It was a magical experience that every boy goes through. Puberty. I have no doubt in my mind you didn't have a personal experience that told you that you were heterosexual.
My point is that maybe he didn't realize he was homosexual until a certain point. It had to be developed by personal experiences.
I had crushes on girls all through elementary school, just because it's what boys did. Once puberty rolled around, and I began to be sexually attracted to people, it flopped the other way around.
Psychologically, everyone 'starts straight', since physically, they CAN'T be sexually attracted to someone until puberty.