Male homosexuality is a major puzzle for evolutionary theory, for if evolution has a purpose it is reproductive fitness, the passing of our genes to our children. This is our ultimate, if unconscious, reason for existence, as pawns in the grand evolutionary game. Sex that is not reproductive seems to make little sense within an evolutionary framework. Reproductive fitness is a simple and elegant theory. Successful species breed robustly. Natural selection weeds out the less fit and the species prospers. Within a species, if individuals who carry genes for a characteristic breed less, they gradually diminish their genetic representation and their special contribution is lost. Following this argument, over successive generations gay men, who presumably produce less offspring than straight men, should gradually be flushed from the gene pool. Unfortunately for such an elegant theory, this is not the case. Homosexuals were with us through antiquity and, if recent history is any guide, are a robust minority within society. So why hasn’t male homosexuality died out as a less reproductive strain of humanity?