Just because your body changes doesn't mean your taste in people would, many trans people report an evolution but almost never going from "I only like women" to "I only like men" or vice versa. A bisexual awakening is not uncommon but for someone to actually lose attraction to the gender they'd previously preferred is a "basically never" scenario. So it only makes sense for that full reversal to be the trope when you unquestioningly assume gender and sexuality to be intrinsically linked and for heterosexuality to be a standard part of men and women. That's why I say they "remain the same," not in the specifics of who they date but in aggregate they represent the same stupid backwards ignorant ideas about what gender is.
And in general you aren't really respecting the huge difference gender makes. A straight man and a lesbian woman do not have the same sexuality just because they both "like women," because same sex relationships and hetero relationships are completely different in form and function in our society. The flirting is different, the stakes are different, the sex is different, the long term prospect of things is different. And the gender difference obviously goes far beyond romance, because social behaviors and fashion and peer influences and all sorts of things are different.