How much does she look like a guy?
I don't just mean in terms physical appearance, but also in action, dress, voice... everything. Do you prefer the way she looks in guy clothing? Are you turned on by her looking like a long-haired man? Then, yeah, you're gay, bi, or possibly just really into mannish women.
If your attraction to her grows as she transitions and gets better at fulfilling her new social and physical role, then you're straight. Gay guys don't want someone who wears women's clothing, women's makeup, who walks like a woman, who sounds like a woman, who acts like a woman. They are into men.
Here's a good, if imperfect, home test of sexuality. Imagine yourself sitting in a restaurant (or, alternatively, go to a restaurant frequented by attractive people.) Every thirty seconds or so, an attractive man passes on your right. At the same time, an attractive woman passes on your left. Which way do your eyes flick when these gorgeous people pass by? If you're a man, and your eyes went exclusively to the left, you're straight. Even if every one of those attractive women were born genetically male, you're still straight, because you want to look at--and be involved in relationships with--women.
Trans women are only men deep down in the sense that, way deep down in their genes, they have Y chromosomes. Otherwise, they act, walk, talk, and often verging on usually look like women. Because they are women. They just got around to being so a little late.
Also, as mari says, lots of of MTFs are still into women after transitioning, and from my understanding, some are sort of asexual. Also, though sex reassignment surgery does create a real vagina, not all trans women can reach orgasm. The surgery's good, but not perfect.