Like many posters before said, i think sex and sexual preference are completely different and determined by a number of factors, even physiology. Let me give you my case which is unusual, even for me.
I was never gay when i lived as a man, i preferred going out with women although it still felt kind of different. For example when i see a beautiful girl first i would compare myself to her instantly and feel depressed, then the Testosterone would kick in and even though your depressed, she is hot so lets go for it.... Testosterone is one hell of a drug btw... The thought of two guys getting it on even now makes me kinda sick, not saying that i have any kind of feelings for people who do that stuff, just not for me.
HOWEVER, after HRT things started to change, i began to lose interest in women and started to be attracted to men. Its unsual, and i cant explain it, but the more estrogen became the dominant hormone in my system the more i was attracted to men to the point where now going out with guys is as natural to me as going out with girls before. The reason i am ok with this is because i look like a woman now as well as feel like one, not having to act like a man, so a guy and girl going out is pretty natural. My BF now is straight, which makes me happy because anything that wanders into the gay zone for me makes me lose interest instantly. He also knows my past, and still wanted to be with me after i told him, which makes him even more special.
So in my case i was straight as a guy, and now I am straight as girl, actually i was lesbian and now straight (since i was always a girl). So you will end up with what you end up, it doesnt matter because either way it will feel normal to you, and thats what is important.