Nature and nurture. Some would say that there is a genetic predisposition, maybe hormone levels during the developmental stages.... But a lot of is it learned. Environmental. Comes from the human desire to be assimilated into a group and mimic others to be accepted. If you look, act and sound a certain way the gay community will embrace you. If you want to be thrown for a loop, go make friends with a gay guy who didn't grown up around the gay community. He will look, act, and sound no different than anyone else. Share similar hobbies as you and more likely than not, you'll get called out as potentially gay long before they will.