Actually, this is one of the reasons I point out to sociobiologists for an evolutionary argument for homosexuality. Most mammals that live in groups have some mechanism for limiting the population, usually by having only certain individuals breed. Given that communal childcare seems to be part of human evolution (for instance, women without children can lactate if they suckle somone else's baby regularly,) it would be pro-survival for a clan to have a percentage of males who not only aren't interested in mating but have more "feminine" behaviour, and are therefore more likely to help with childcare.
I don't actually believe this theory, because it depends on assuming that stereotypical "gay" behaviour is genetically determined. But sociobiologists do believe that such minutiae are not only genetically programmed but have survived unchanged since we were cracking flints in Olduvai Gorge, and it amuses me to turn their logic back on them.