I noticed I was bisexual before I admitted that I was trans. In fact, it was through introspection about my bisexuality that the light finally began to dawn about my transsexualism that I'd been hiding from. The puzzle: I feel attracted to men... but when I looked at gay men and tried to imagine being one, I just couldn't. Contemplation of this insight uncovered my deeper inner motivation: I needed to have a vagina. This finally led to conscious understanding of my unconscious gender, to use Julia Serano's apt term. I wasn't a gay man because I just wasn't a man. My bisexuality was brought to light by the force of my inner female gender as it grew stronger.
From my experience, I can understand the apparent shift in sexual orientation that is often facilitated by HRT. I had awareness of my bisexuality long before I started hormones, but the feelings that the hormones developed in me just made it easier for my sexuality to find its own level.
Sexuality and gender, while different, are nevertheless intimately interconnected: Although I felt sexual attraction toward men before I came out as trans, I had no way to actualize that feeling-- because it was heterosexual men I felt attraction to. Expressing my gender as a woman allowed me to actualize my attraction to heterosexual men. A large part of their appeal for me is that they conform with my womanhood. My lack of sexual attraction with gay men was not that they're gay (I'm queer all right) or that they're men (I'm attracted to men)-- but that if I hooked up with a gay man, it would mean he saw me as a man, and being seen as a man always caused me intense suffering. I'm attracted to heterosexual men not because I think heterosexualism is anything wonderful in itself, but because of how they are attracted to me as a woman and provide validation for my womanhood. Likewise, I'm attracted to lesbians because of my womanhood, while heterosexual women bitterly disappoint me when they express sexual attraction to me as a man.
Bottom line, it's all about my womanhood. The only people I can hook up with are those who relate to me being a woman. Anyone who tries to relate to me being a man will get nowhere because I'm not a man and I resent being treated as one.
I wonder if this thread's originator is new and inexperienced at trans life. For some months after I came out, I had a "male mode" at times simply because it took a period of adjustment to grow into such a major life transformation as being an out trans woman. But as soon as possible, I dropped "male mode" entirely, and never went back. All I care about is living as totally as a woman as possible. After nearly three years, I'm finally achieving that goal.