I'm afraid osmosis only goes so far. I was raised around heterosexual people, and I myself am one. When I later met and became friends with homosexuals, it didn't stir any confusion in me. Sexual desire is a strong emotion, and I know what rings my bell. Similarly, when I roomed with a girl from Africa last semester, I don't recall ever looking in the mirror to see if I was still white.
Sexuality is biological, just the same as skin color, eye color and so on.
My parents could have moved to Indonesia when I was born, and I can guarantee you that I would not have developed any of the physical characteristics indigenous to people of Asian descent. I realize that this analogy might be lost on some people. After all, behavior and physical appearance are two completely different things. Ignoring homosexuality does not make it go away. Based on your own arguments, this implies that it is, in fact, not a choice.