I would say that I have always been interested in and fantasied about men. In that sense, my sexuality has been the same my whole life.
However, prior to HRT I was a lot more sexually attracted to women. I wasn't interested in them however. If that makes any sense. But I couldn't help but feel aroused by attractive women, however all it made me want to do is have sex with men. I was only really interested in having a girlfriend as part of going through the motions of being male. I was depressed as a male, I felt like I just had to comply to the role (Which meant being gay wasn't an option, to me, at the time). And the idea of having a girlfriend seemed so dysphoria inducing that, it was never something I actually pursued.
I used to feel like I must be nuts and wonder "is everyone else like this?", because I could NEVER feel comfortable with the idea of having sex with women. I could be aroused by them, but that was it.
With men however I have always been interested in having sex with them, but only as a woman. The idea of being with a man as a man seemed unacceptable to me (nothing against gays, but it wasn't the role I was trying to play at the time).
Since starting HRT... my attraction to men has remained exactly the same, however my attraction to women has largely vanished. Which I am happy with, being attracted to women was something that confused and frustrated me.
The funny thing is that, my whole life my sexuality has been mostly indistinguishable from a hetrosexual female. I have always been interested in having sex with men as a woman. When I was still attracted to women, I didn't really want a relationship with them apart from going through the motions of being male, keeping up the act i was playing. And I didn't really want to have sex with them. However despite that, when I was trying to be a male, I thought I was straight then too. I wondered if maybe everyone had this frustration, I didn't know. I was very confused and didn't really want to face that confusion.
Hard to explain I guess.