I can't speak for anyone else, but this is how I've sort of worked it out for myself.

My first "sexual" (or at least "sensual") kiss was with a boy. We were nine, or at least I was; he might have been ten. We did it on a bus ride to a summer day camp, his idea; I figured the intention (being nine) was to "gross out" the rest of the bus which I thought was a great idea. But I found I quite enjoyed the sensation, even without all the amusing expressions of general disgust that "smooching" elicits from eight- to twelve-year-olds, and was mildly annoyed at the exclamation of "ewwww, ho-mo-sex-shoo-walls!" from some older kid in the front of the bus. Not that I really understood what that meant beyond the literal meaning, but it was jarring to realize that others really did see us as two boys kissing, even though that was kind of the point of the exercise at the outset, because that's not how I experienced it and it kind of ruined the moment for me.

As I grew up, I mostly didn't question the sexuality of my friends, though I tended to gravitate to people who weren't too stereotypically masculine or feminine just because I guess I felt more comfortable with them. I formed emotional attachments equally easily with people of either sex. I only ever found myself fantasizing about sexual situations with any of my male friends when I was imagining what my life would be like if I had the female body I felt I was supposed to; the idea of a boy being attracted to me because I was a boy wasn't exactly revolting or anything, but more disappointing because it missed the point. But the idea of me being attracted to boys who were attracted to me as I saw myself deep down, as a girl, wasn't a problem for me emotionally. And the idea of a boy being attracted to me as a girl gave me a happy, warm, comforted feeling. I could get aroused thinking of myself as a girl being with a boy, but I never could get aroused thinking about myself as a boy being with a boy.

Girls of course were a whole different matter. Mostly I was suffused with envy that they got to have the bodies they were supposed to and I didn't. When the testosterone started flowing and stirring up sexual feelings I spent a lot of time imagining what it would feel like to have a body like that, and to be touched, caressed, and so on. I also "knew" that as a boy I was expected to be with girls, so I tried to imagine what that would be like. And it was quite arousing, in contrast to imagining me as a boy being with boys. It was confusing when I tried to understand it, which I didn't too often, but eventually I realized that what was so arousing about it was imagining what the things I did would feel like to her. But when I imagined having a girl's body myself, imagining being with a girl wasn't nearly as arousing. Again, not revolting or anything, just--not sexually exciting, particularly.

So to an outside observer, I suppose I looked rather like an ordinary heterosexual male. (Then again maybe not; I've always tended to set off people's "gaydar" for some reason.) I suppose my strong ability to empathize with women has made it easier for me to "fit in" that way and probably helped me be a sensitive lover (a compliment I've gotten from more than one partner). Whenever I've been in a sexual relationship with a woman I've found it quite possible to get aroused thinking of myself as a woman with her, so at times I've wondered if maybe I was a "lesbian trapped in a man's body," but then I've remembered high-school daydreams about being a girl and "making out" with one of my male friends and decided that didn't make sense, or at least couldn't be the whole story.

Other times I've wondered if I wasn't just a gay man in deep denial, but the idea of being gay doesn't bother me as far as I can tell; I've actually felt quite bad more than once at having to disappoint friends who were hoping I was, and with one close friend even tried "making out" a couple of times just to be sure but it didn't do anything for me at all, even under the influence of a few wine coolers to loosen any inhibitions I might have had. I suppose I could "be" gay and just didn't find those particular guys attractive, but it just doesn't feel like a good fit for me, so I don't think that's it either. And for all the aforementioned reasons I don't think I'm exactly bisexual, either.

Anyway I try not to worry about it too much, but when I feel the need to categorize my sexuality these days I tend to look at myself as a more-or-less heterosexual woman stuck with a man's body, but with certain useful coping skills built on strong empathy and a vivid imagination.