Although I'm pre-everything and therefore can't say if HRT will have an effect on my sexuality, I have noticed a shift in my own perception of my sexuality since coming to terms with the fact that I'm a trans guy.
I've always identified as bisexual, being more physically attracted to women and (I thought) more emotionally attracted to men. As silly as it sounds, the reason I thought this was because I became intensely attached to many male fictional characters. I loved writing the part of men, whether it was in stories or roleplaying games or whatever, and I led myself to believe that my ideal partner would be this certain type of man that I always gravitated toward writing/reading about.
Over the years, I realized that I wasn't attracted to that type of man, I wanted to be that type of man. I realized that even if I met this ideal guy, I wouldn't be fulfilled by dating him. I would feel empty and jealous that he got to be the person that I always wanted to be.
Roleplaying also taught me that my sexual preference for women was as a man and not as a bisexual woman or lesbian. When I was writing from the POV of the man and developing a relationship with a female character, something clicked. Everything, including the idea of sex, felt so much more right when I viewed myself as male. I discovered that I wanted to be a girl's man, boyfriend, prince, Romeo, dream guy, husband (someday), all that stuff.
Upon accepting myself as a guy, I have also become more comfortable with the idea of being with another guy. Back when I was trying to be female, I dated men and had sex with men, but it never felt right at all. I can much more easily see myself dating a man as my true male self, but it still doesn't appeal greatly to me. Nothing feels as right as the idea of myself as man with a woman, romantically and sexually.
Another note: no matter how appealing I found someone before, it was hard to be truly attracted to them when I didn't like the idea of myself performing as a female sexually with them. Visualizing myself having sex as a female with almost anyone was a big turn-off. It always felt wrong. So I was very prudish and uncomfortable with intimacy and even thought myself asexual many times. But now that I see myself as a man and am going to start transition, everything has changed. I view sex as a wonderful thing that I'm very excited to have now that I can, or will soon be able to, perform as my true self.
Overall, I think it makes perfect sense for our sexualities to flux as we transition. A big, big part of sexuality is the role that you play in it yourself, after all, which I think people forget sometimes. When most people talk about sexual orientation, it's "do you like men, women, or both?" But something that trans people know better than cis is that it doesn't end there. It also includes "do you like the idea of being a man with a woman, a man with a man, a woman with a woman, or a woman with a man?"
