When you break sexual behavior and preferences into just that.
Labeling people an X, Y, or a Z mirrors our modern and post-modern obsession with identity, rather then the sex practices themselves.
If an otherwise homosexual man has sex with a woman, and likes it, does that shift his "category" to bisexual?
Does it f-ing matter?
Yes, if he stays with her it becomes a category shift, in that certain "gay" venues become closed to him, and "straight" ones become accessible.
All of this, because he is now having vaginal sex?
But this is not universally the case, it becomes so in a modern state-level society, with all of its trappings. These institutions and laws and assigned rights, have to be in place, to begin with.
In a society without a church or a gay dance club, we get back to what people are doing, instead of who they are. Some of us will prefer to have sex with other women or other men, some can make it work either way, and others will have no interest in getting it on with another man or another woman.
Is sexuality malleable?
In a certain sense, yes.
Are religious dogmas, culinary preferences and career choices malleable?
Sure.
Basically (vague, i know) "studies" have shown that people can get used to it and learn to like it, for the most part. Key word: adaptable.
If circumstances require it, most New Yorkers can learn to like living in rural Nebraska, most raging bitches can learn to tolerate their cunty in-laws, most gays and lesbians can find some kind of partner of the opposite sex tolerable.
At some point, you just kind of find ways to like it, because it sure beats being fucking miserable all the time.
Not advocating "conversion therapy" (forcing another social construct on somebody).
Not everybody is adaptable. And your new circumstances come at a grave sacrifice because they go against the grain of what we have grown to find comfortable.
Does getting HRT make an otherwise or previously "heterosexual" XY want to swallow cum?
Maybe. But like somebody else said, this could be part of their "being a woman" fantasy. In essence, a socially-constructed interpretation of how an XX expresses her sexuality.
Frankly that urge is more of a reflection of male-oriented pornography then it is of female sexuality, because a lot of women find that practice to be degrading and vile and don't choose to engage in it.