I think that language is tricky in this case... AFAIK, and that obviously includes some doctors and researchers of social sciences, there are no 'common' cases of people having their sexuality 'reversed' after transition. There might be a few, sure, but in those cases, it will be very hard to say if the person wasn't bisexual in the first place, just never admitted it (not even to themselves, much less to their doctors); and because sexuality is also a spectrum, someone might start as being 99% hetero and 1% homo, and, after transition, getting curious about that 1% they never really got any chance to explore — and having the 'appropriate' body to do so now, they might actually enjoy it, for the simple reason that they might not have enjoyed sex before transition anyway. In other words: I personally believe that almost everybody is bi-curious to a degree :) (I'm ready to admit there might be exceptions), and transition might just be the trigger that releases that bi-curiosity and drives the trans person to some exploration...
Here is a classical example: a guy in his 50s always thought of himself as heterosexual, but he also always knew that he was in the 'wrong' body. Sometimes, the gender dysphoria is not critical enough to push someone into transition (see Caitlyn Jenner). Instead, he had a normal amount of girlfriends (which, perhaps surprisingly for many people :) is a far lower number than it is usually claimed...), enjoyed sex with them but not overmuch so, even though, as a trans woman, this guy appreciates female beauty in many more ways than one, and having sex with a woman is very exciting for that reason. In his mind, however, he might actually imagine himself as a woman having sex with another woman; in other words, while his sexuality might be a bit confusing, there is no doubt that he is a genophile, i.e.attracted to females (or at least to the female body). He never has an experience having sex with a man; for several reasons, this is actually a disgusting thought to him (but exactly why, he cannot tell).
He does his service in the Army, gets a job as a salesman or truck driver, finds a wife, has kids. And suddenly, when reaching what was formerly known as 'the midlife crisis', he starts to question a lot of things, the first, of course, being his gender identity. Eventually, with the help of doctors, he's diagnosed with gender dysphoria, which has been repressed over decades and never had a chance to manifest itself; but during that 'midlife crisis', men are a little more vulnerable, and prone to insatisfaction about their life in general, so eventually he stops pretending that he is 'male', and begins his transition, hoping that at least he might live to the end of his days in the 'right' body for his identity.
His wife may or not may be accepting (very likely she'll divorce him — this will happen in the vast majority of cases), but he — now she — is still attracted to her. The physical and mental attraction to women has not changed, and she didn't expect it to change. She starts looking for potential female partners, with some degree of success or not, and even after SRS, her desire, both physical and mental, is still 'women only' — she's still a gynophile and being a gynophile is the kind of sexuality that she has always had, so it does not feel 'different' in itself, except, of course, that she finds out — the bad way! — that if it's already hard for a lesbian to get a partner, it's even harder for a trans lesbian to find one!
At the same time, however, she also finds something else (and this is actually not very surprising, and depends quite a lot of the kind of preparation she was given by her therapists). A few of her best male friends, after the initial shock, have still kept their friendship (most have abandoned her). She still keeps 'going out with the guys' — perhaps not to the same spots, perhaps not doing the same things as before (until it's clear that both men and women are perfectly able to enjoy a night out on the bowling alley drinking beer), perhaps with a few hesitations. But those who are true friends will eventually continue to be friends no matter what, and give whatever support they can. Some might even be drawn closer to a friend in need; some might not want to be so close due to the 'weirdness' of the new situation, but still be supportive, and continue to be in touch with a similar regularity as before. What happens next is that this trans woman suddenly realises that not all men are jerks — one typical description made by tons of trans women before transition. Sure, yes, the majority are complete idiots, but there are a few that are not, and who are able to think well beyond girls, cars, beer, and sports. In fact, she now realises that such men are the kind of men that she already had as friends before transition! And she further realises that this should not have come as a surprise: we tend to gravitate towards those that we have something in common with. A very repressed trans woman will still have some traits which we associate with 'femininity' in her personality and tastes, even if she tries to repress them as much as possible under the cover of a 'macho man'. But the truth is that there are no inherently 'male' or 'female' traits — there are just 'traits', it's society that defines which ones are assigned to one gender role, and which to a different gender role. In any society, no matter how stereotypical the gender roles are enforced, there will always be men with feminine traits, and women with masculine ones; this is unavoidable, because... traits are just traits, it's just us that label them (I know that some authors still consider that there are special traits, like, say, affection and care for children, which women are supposed to have in a higher degree than men because of the strong bond between mother and child, and while I can accept the idea of the 'stronger bond', saying that women love their children more than the father is, well... sexist :) It's just that fathers are socially conditioned to express their affections towards their children differently, not that somehow their 'feelings' are attenuated or not so intense... I personally refuse to believe that, and, as far as I know, there are no easy ways to determine the intensity of a 'feeling' or 'emotion' based on a standard scale or something like that — because we humans express those feelings/emotions so differently and with so many different intensities. Also, such intensities also vary with time and are changeable — that's the root not on of Buddhist training, but of many therapies to cure so many mental diseases. But... I totally digress :) )
So, uh, to come back to the point: this trans woman, perhaps for the first time, now understands why women in general have such a difficulty in finding interesting partners, and why both women and trans women can universally agree that 'men are jerks': it's because in our current Western society, men are much more conditioned to fit to a stereotype than women. We gynophiles find women 'more interesting' because they are 'allowed' to display a much vaster variety of emotions, feelings, and interests than men; that means that they are able to establish conversations about pretty much everything, while men are 'expected' to limit themselves to a handful of choice topics which are 'appropriate' for men. Geeks defy these conventions and focus themselves on utterly alien topics — for both males and females! — but I also think that the 'rise of the geeks', which happened in the 1970s or so, comes mostly from a suppression of the number of interests that a man is allowed to have. Basically he's supposed to be focused in his work, and his goal is to have success, i.e. gain money, power, influence, fame, reputation, and so forth; anything else are 'distractions', some of which are 'allowed' — sports, cars, women — but most are not. In the past, of course, it was artists who defied the status quo; geeks are those who also defy the status quo but have not talent enough in 'classic' art to be able to become artists themselves (though many do!). Anyway... my point here is that it is not uncommon for some trans women to find out after transition that her male friends of her pre-transition phase are those that display many so-called 'feminine' traits, which she obviously also has, and that what was brought them together. This is not universal, of course, but I found out (by meeting in person some of the male friends of a few of my MtF transexual friends, who 'came out' to them before or during their transition) that it is much more common than I thought, and I haven't seen it mentioned so frequently in research papers or among the community, mostly, I think, because the vast majority of trans people still cut themselves from all their past and start a new life under a new name, identity, city, job, and so forth, and often never turn back to their past.
Among those who do not cut with their past, however, I find this trend of having male friends with characteristics which we would call 'feminine' is actually quite frequent. Note that I'm not saying that all those friends are themselves also trans; not in the least! I'm also not saying that they are 'effeminate' — again, this will very likely not be the case, because MtF trans people, before they even think of transitioning, will often shun and avoid those males who do not look male enough, lest they are themselves found of 'less-than-adequate-maleness'. In other words: in many circles, being male and having a male gay friend — friend, not lover! — is still misunderstood: it is often assumed that if you 'hang around' gay males, then it's because you're gay as well. I know, I know — stupid but true — but the point here is that MtF trans people who repressed their 'true' selves for ages will almost always avoid the company of those males who 'do not look male enough', in order to assert their own maleness and thus avoid suspicion.
Nevertheless, one thing is the appearance (or even the sexuality), the other thing is the personality. The most incredibly macho type, who is really 100% heterosexual and makes sure to look like it, may nevertheless have some traits which we would consider typically female: to give a stereotypical example, men also likes poetry that make them cry (there are more male poets than female ones), so what does that tell us about the sensibility, the range of emotions, etc. that men are supposed to experience 'just because they're men'? Men cry as much at emotional movies as women; the difference is that they disguise it much better, for fear of showing themselves 'less then men'. I have in my mind one movie, a comedy, with all sorts of mobsters with Italian accents, who are all tough and macho, but their leader, who is the toughest and macho-est of them all, is always showing himself of having all sorts of 'female feelings', often even shocking his fellow mobsters — but because he's the strongest, toughest, etc. guy around, they do not dare to point out to the Big Mafia Boss that he's 'behaving like a woman'. I can't remember the name of the movie, or of the actors (I'd guess Al Pacino), but I found that the scriptwriter was very clever and ingenious to show how tough and macho a Mafia mobster can be, while still having emotions, feelings, behaviour, and a full personality that, under the cover of male bravado, is actually much more similar to a female personality — something which is also cleverly shrugged away in the movie since, well, we all know that stereotypically all Italians exaggerate in their emotions, even men, right? So Italian men can cry when reading poetry, because, well, Italians are the epitome of 'emotionality'. If the movie's setting was different (i.e. without Italians!) then it would look much stranger... even if there would certainly be lots of men identifying with that character! (Italian or not!)
Anyway, and this answer is already huge, and I haven't come to the point yet! So, my hypothesis here is that trans women before transition may have several acquaintances of the 'macho men' variety, mostly to divert suspicions, but their true male friends, those with whom they share a special bond, will very likely have much more so-called 'feminine' traits in their personality — even though they might consider themselves the macho-est of men and also engage in all sorts of activities that are 'typically male' and even enjoy some of them — and this is the main reason why they are drawn to each other. Even ordinary, cisgender, heterosexual males who share some feminine traits will come together as a group of friends; and this is much more frequent, even though most men don't even realise why or what they have in common with each other that makes them such good friends. The expression 'BFF' comes from the women's world; men are not supposed to have that close a relationship; but of course a lot of them do, and if such a friendship is not as 'close' as two BFF women, it's mostly because of social restraints that prevent men to be that close. Remember that I'm not talking about sexuality; in exactly the same way that two women can be BFF but never ever dream of having sex with each other (or with any other woman), the same happens to men as well — closeness does not imply sexual attraction. However, I would also believe that it might be more common among men with many feminine traits to share a more fluid sexuality, similar to the kind of fluidity that we are starting to accept to be normal among women; again, do not fall in the trap of generalisation, the whole concept of 'BFF' has always existed (even if not under that acronym, of course), both among men and women, and to a degree, the close bond of friendship among men is somehow tolerated, within limits. It is not surprising that literature before the rise of puritanism and the current system of gender roles will show men having 'best friends' described in terms that we would today employ only for female-to-female friendships, and which are not acceptable for male-to-male friendships.
So, uh, what has all that to do with the trans woman we left in suspense a few paragraphs ago? :) Well, she will now realise that those guys who are not jerks are, in fact, either her 'best friends' before transition, or, well, men that she will meet who have strangely similar characteristics to those friends. In fact, she will figure out (like actually all women also do!) that there is a class of men who exhibit several traits that she, as a trans woman, recognises in herself, and knows to be feminine; she might not have been aware before how other males — males who are cisgender and heterosexual — also exhibit those traits. And, because of that, she now realises that the bonds she shares with women are based on several mutually common traits, typically associated with women, and that's why it's comparatively easy to forge new friendships with other women, they have so much in common after all; but some men, unlike what she thought before, also have the same traits. And it's therefore 'natural' that she finds those men... interesting.
Now, I'm talking of 'interesting' as in: 'having an interesting personality'. So, in spite of her androphobia (distaste, hate, even fear from those who are male), she might be open minded enough to accept that some men are indeed 'exceptions to the rule', even though it is even more intriguing that such men are not trans at all, not even slightly; they are also purely heterosexual, and, while they enjoy strong bonds with other male friends, they would never come to the idea of having sex with them. They might not even be bi-curious whatsoever — as said, personality traits are just 'masculine' or 'feminine' based on social criteria, and they have absolutely nothing to do with sexuality... — and simply reject that idea as total nonsense. They will not brag much about their sexuality, though ('bragging' about one's sexuality is one of those 'stereotypical male traits' — one that, of course, a lot of women also have!), even it's crystal-clear that they had no experiences with same-sex intercourses, and have absolutely no interest in those.
But the trans woman, as a lesbian, now faces the problem that all lesbians have to painfully endure every day: this heterosexual guy is looking at her as a woman; they share similar interests, traits, emotions, feelings; they enjoy the companionship among themselves; being heterosexual, however, it'll be not unlikely that he'll find her attractive — first in a purely mental, platonic way; but, who knows, the desire may also become physical at some point.
Because such things are never easy, here is what happens in the trans woman's mind: she is still attracted, both physically and intellectually, to women. She knows that very well. She has no interest in men whatsoever — because 'men are jerks'. She might even have a strong aversion to the male body in general — mostly because it reminds her of the body she had to drag around for decades of silent suffering. And while she might have the occasional dream of having sex with a 'faceless guy', like so many MtF crossdreamers are fond of describing, one thing is a dream, the other is reality: she does not harbour a 'secret fantasy' of having sex with a guy! This has been always crystal-clear — and even the source of much discussion with her therapist — and there is nothing about it that she can do, you don't 'change' sexuality, just as you don't 'change' gender identity, both are inborn characteristics.
Nevertheless, as said, this particular guy is 'intriguing' because, well, he just looks like one of her former male BFFs — not physically, but in his personality, which has so many feminine traits that it baffles her — especially because the guy himself not only doesn't realise that, but, even if such traits are pointed out to him, he just laughs, or shrugs and disregards her remarks as being utter nonsense. So, yes, he is interesting, he is intriguing, and... he's definitely also lusting for her, which is a funny thing to happen to a lesbian trans woman!
Now, at this exact point, is where I postulate my hypothesis, that this trans woman has always been bi-curious, she just never thought about it, nor labeled herself as such before. Or she might simply have always been pansexual — attracted to people's minds and personalities irrespectively of their physical bodies and sexual characteristics — but because pansexuality is not widely talked about, she might not consider herself something that she has never heard about. What she may think is that while having sex with another guy while having a male body with male genitalia was utterly impossible, now she suddenly realises that she has 'the right package' for this particular guy. In other words, he's just expressing a perfectly normal heterosexual desire towards her. And she is now not only a woman in mind (which she has always been) but also in body; so it's very different of thinking about 'sex with a guy' when you're equipped with a female body (or as close to it as medical technology can get you) than having 'sex with a guy' when both have male bodies with male genitalia.
In fact, just out of curiosity, she might wonder how it would feel like to have intercourse as a woman — while before transition, when she still had a male body, the idea of having male-to-male intercourse was absolutely repugnant.
This is not to say that she has 'changed sexuality', or even that somehow she has 'expanded' her sexuality to embrace bi-curiosity (and, who knows, even bisexuality or pansexuality in the future). No: the point is that there was the potential of bisexuality, or at least some fluidity of sexuality including bi-curiosity, rooted in her mind. But such potential was never tapped because her body was the 'wrong' one — which meant that she focused on an act that was repugnant to her (male-to-male intercourse) and therefore rejected it, while, at the same time, she wasn't even aware that her intellectual attraction was to certain traits that usually are (stereo)typical of women only, but that she now understand to be present in men as well. In fact, pre-transition, it is quite likely that her best friends, male or female, had similar traits; it's just that she could not imagine intercourse with a male while having a male body, so she focused only on females instead — and this reinforced the notion that she was purely gynephiliac (and totally androphobic), which was also what she had told her therapists over and over again: she is only physically interested in women, never in men, so she was aware that she would become a lesbian trans woman after transition, and that was exactly what she always wanted to be seen as.
What she didn't count on was that the physical change of her body also triggered this 'potential' that had been locked inside her, and which had absolutely no way of expressing or manifesting itself, because the male body simply made her too self-conscious about her own physical masculinity; once she got rid of all that, yes, there was a change: something which has always been there — a propensity towards bi-curiosity or even bisexuality — has now been unleashed.
Oh, BTW, I should have read that Playboy article before writing this post, but I didn't want to be influenced by it :) Still, in one of the comments, I found a message which could have been written by one trans woman friend of mine. She also thought, for a while, that she somehow had to 'learn to like men', and when she told me that for the first time, I laughed, because I thought she was joking. Then I saw that she was being dead serious, so we talked a bit more, and I told her that this would be highly unlikely — she was always attracted to women, and hormones will not change that. But she was not totally convinced; she said that she would 'have to be open-minded in case my sexuality flips'. In other words, she was worried that this might be the case, and that she was totally unprepared for when it happened, and that generated some anxiety to her. I tried to keep her off those ideas, it's stupid to be worried about one's sexuality 'flipping' — she clearly had read some articles such as the one from Playboy — but it was pointless. In fact, it was only until she had been taking hormones for about a year or so, and fell madly in love with a cisgender woman, that her fears were all gone — she definitely would continue to be attracted, both intellectually and physically, to women.
The article, however, shows many examples of bi-curious or bisexual people who had a special circumstance that 'triggered' an extension of their sexuality, by unlocking a potential that most assuredly had always been there in the first place. The case of the trans woman who never in her life thought about men, but had a date with a man who was particularly 'interesting', and eventually fell in love with him and was still a bit confused about the physicality of her relationship with him... well, this describes pretty much some cases I had read about elsewhere — it's much more about recognising that some men are not complete jerks, recognising that some people are attracted to certain so-called 'feminine traits' (which men also have, and suppress more or less, depending on their social conditioning), and that this, in turn, will trigger the realisation that, after all, one's sexuality might be more flexible than one had imagined... well, that sounds about right. There are lots of situations that will 'trigger' one's realisation about one's sexuality. I remember so many cases in my life of guys who have always considered themselves to be the total embodiment of pure maleness, and, when talking to me for the first time, even knowing that I'm really just another male beneath all the layers of my shapewear, wonder why I have never 'experimented' with my sexuality and tried to have sex with a guy before; and most fully admit that they never think of themselves as 'gay', much less 'homosexual', but merely as something that they 'experimented' and that they found out that it wasn't as horrible as they thought it would be; so, they insist, I should also keep an open mind and 'experiment', maybe I would 'like' it if I had a 'taste' of it?
Well, I usually reply to that that I don't need to 'taste' everything to say what I like or what I don't; just because millions of flies look at a pile of shit and consider it to be a delicious meal, that doesn't mean that it will be as delicious to me; I can know in advance that certain 'experiences' are not a matter of 'taste', as those people imply. On the other hand, they — and many others — still believe that being a crossdreamer or a transgender person is a question of 'choice', something that we 'enjoy' doing because we 'experimented' with it and saw that it was 'fun'; to them, I just tell them the stories of a handful of MtF transexuals I've known who had never out on female clothes or makeup before their transition, because they saw no point in doing that over a male body; so it's not as if they had 'experimented' being women, found it nice and fun, and then went ahead chopping some bits off their bodies and adding a few extra. It's not how it works. Those things are simply hard-wired in our heads; it just takes a trigger to unleash them. In my own case, I never really put on women's clothes before I was 25 or so; back then I had no idea what 'crossdressers' were, and the whole notion of 'transexuality' was very confusing, and it would not 'fit' me anyway, because I assumed that all MtF transexuals were attracted to males only, and I wasn't, so I couldn't be one :) So... it was a sequence of circumstances, many by chance (the availability of some lingerie from one of my female cousins) that triggered the sudden, strange urge to put them on, and then lead me to discovering a part of myself that I had hitherto repressed to the deepest levels of my core identity. But it was always there, lurking to come out; it was not something I 'experienced' and said, 'oh, I have to be open-minded about it, perhaps wearing women's clothes is fun, I cannot know if I never try!' No — that's simply not how it works! (and, in the beginning, wearing women's clothes was not exactly 'fun' at all, it was just something I needed to do to somehow find some mental relief from a tension that had always been there — and still is, but at least now I know what it is and how to deal with it... to a degree! :) )