Sickle Cell Anaemia as a Model for Homosexuality, by David Archibald.

Homosexuality is a conundrum. Theoretically it should be bred out of the population rapidly but instead it persists. There is plenty of evidence that homosexuals are born that way and therefore are part of the human condition. Homosexual males tend to have a longer index finger than middle finger (as heterosexual females do), with the opposite in lesbians. Birth order affects the incidence of homosexuality, with it increasing with the number of males born to a woman.

So why does humanity include a variant that makes them less likely to reproduce?

Read on at the link.