I mean no offence at all to Zombie_leech, as he is definitely not alone in being unsure or ambiguous about his preferences. But for me there was never any doubt. Both physically and emotionally, it's so clear to me that I'm attracted to men, from as long as I can remember.
Physically, and this is the most obvious of the two, I get aroused and my dick gets hard from the male sex, while the female sex does nothing for me. Can't get much clearer than that.
Emotionally, I mean, being smitten by someone, or getting a crush on someone. In this case, occasionally I can find a woman very attractive or interesting, in the same way straight guys can develop a "guy crush" on their favorite celeb or whatever. But no girl could ever make me weak in the knees, faint in the heart, light in the head, ... like certain guys can. It's such a powerful emotional reaction, that -if someone were to have this-, it must be a clear sign of "who" they're attracted to.
Soooo, if we go by the idea that sexuality is a spectrum, I'd place in the "obviously pretty damn gay" category.

I know this doesn't really help anyone on the subject, but I thought I'd share.