Why are some men born gay?

A hypothesis:The object of human sexuality is to make more humans. The more intercourse, the more pregnancy. But pregnancy is dangerous, both for mother and infant. In order to encourage sex, we humans are equipped with erotic "triggers" that cause us to find others sexually attractive: hair, body build, pheromones, face -- just about anything. From "nature's" perspective, there's no problem if our sexual urges "spill over" to homosexual encounters.

Homosexuality also serves at least two key social functions. It enables youngsters to learn about sex without causing pregnancy, and it serves as an outlet for sexual urges when the community needs to limit population growth. (Studies in rats have shown increased homosexual activity when the colony becomes crowded, even if there's plenty of food and water available.) If this hypothesis is true, the ability to have homosexual intercourse is actually a survival tool.

There is an hypothesis that sexual orientation (at least among males) depends on how much testosterone the fetus was exposed to, and when.