Sniffing Out the Gay Gene by Pinker
Homosexuality is a puzzle for biology, not because homosexuality itself is evolutionarily maladaptive (though no more so than any other sexual act that does not result in conception), but because any genetic tendency to avoid heterosexual opportunities should have been selected out long ago. Perhaps "gay genes" have some other compensating advantage, like enhancing fertility, when they are carried by women. Perhaps the environments that set off homosexuality today didn't exist while our genes were being selected. Or perhaps the main cause is biological yet not directly genetic, like differences in hormones or antibodies that affect the fetus while it is developing.