Darwin hypothesized, you see, that normal sexual practice involves males competing for female partners in order to spread their genetics through breeding as much as possible. This theory would mean that all non-reproductive sex was aberrant and wasteful, much as the Bible would have it.
However, Roughgarden postulates that Darwin was incorrect in basing his theory on some very narrow observations and that, in truth, sexual behaviour among higher vertebrates is as much for social bonding - which leads to more successful survival rates of the species - as it is for straight-up fertilization (pun fully intended). Her evidence? Over 450 different vertebrate species engage in homosexual sex. That's right: 450+ species that we know of (so far) are actively homo or bisexual, sometimes on a regular basis. Kind of makes you think, huhn?
go read it right now.
Alternately, you could read this article on the same topic, which is much funnier.
[Tip of the hat to Joe.My.God.]