Scientists think they may have found the 'gay gene'.

Homosexuality in males may be caused in part by genes that can increase fertility in females, according to a new study.

...his team hypothesize that the genes they modeled may cause people of both sexes to be extremely attracted to men, which would lead men with the genes to pursue relationships with other men, while causing women with the genes to have more sexual partners, and become pregnant slightly more often than an average woman.

This doesn't explain lesbianism though

"We're still working on lesbianism, but were not getting to the same result, and possibly we'll come out with a completely different explanation," he said.

I've got an explanation - chicks are hot!