So, just to dig into this question a bit more deeply...
Why is "orientation" and "sexuality" so often defined in terms of who we are attracted to? It's a very objectifying construction, but more obviously, it doesn't really take into account all parties concerned. If someone with a penis is attracted to someone with a vagina, we typically call this heterosexual, though the person with the penis, identifying as female, might consider herself lesbian. And then all these changes happen, not the least that she gets a vagina, but now she discovers she's no longer interested in sex with women, she wants sex with men, or at the very least with penises. And now the sex once more is heterosexual, though some others -- believing that birth assignment takes precedence -- would say that she's gay, though she'd probably only agree if her partner was a pre-op or non-op transwoman.
Even if you attracted to same sort of person before and after transition, if you change sex you're also changing your orientation from gay to straight, or vice-versa. And likewise, if the people you're attracted to switches up through transition, you end up still being straight or gay. Which is to say, something or another is going to change. Because you are changing in a deeply fundamental way, at least in terms of how we categorize sexuality.
Not to say that there aren't orientations that can't stay rock solid. If you're asexual before, and asexual after, no change. If you're pansexual before, and pansexual after, no change. But these are all or nothing sort of cases.
In the end, it's impossible to predict how your sexuality will turn out after undergoing transition. Transition is all about change. You won't really know what you're like until you come out the other side. Best, I think, to just be open minded about it, because I don't think it's really something we get to choose in terms of our interiority. Which obviously should be a pretty familiar feeling to most of us here.
