I feel like I can remember the exact summer - between seventh and eighth grade - that girls stopped being the approachable friends that I was on generally better terms than with my male classmates (I liked their games better) and became alien creatures that it took me three years to learn how to talk to, again. If I recall correctly this was associated with an enormous change in my height.
That this was heterosexuality never seemed to be in doubt, I guess. I have little trouble believing that the same thing happens to gay people, too. Maybe not every one, and maybe it comes with a substantial amount of self-denial in some cases, but there was just nothing voluntary about sexuality "kicking in", and nothing ambiguous or obscure about it. I immediately knew exactly what I wanted to do with, and to, my female classmates. Prior to that, the notion of sex - the mechanics of with I was introduced to by an Osborn book on reproduction and pregnancy - was something I was completely indifferent to. Nothing but an example of adults being weirdos. The boundary for me was just incredibly sharp, I guess. I don't believe my experience is universal, but I'm here to tell you that as soon as sex was imbued with its strange and terrible power to hold my interest, I knew immediately who I would want to have it with.