There was a brief discussion on Signel a while ago on the biological causes of homosexuality and I posted this.
I think that homosexuality survives only BECAUSE it aids a particular species. Otherwise, it would been discarded by evolution long ago. Someone posted an article a while ago on the theory of homosexuality's role in strengthening social bonds. Although that is true in many animals, e.g. bonobo chimpanzees, I'm personally not quite sure this is the primary reason, or was it simply a evolutionary development that got tapped on for other uses. (This is not uncommon in evolution - scientists postulate that wings of birds evolved first as a tool for heat regulation, for example.)
Those of you who have read Richard Dawkin's Selfish Gene would be familiar with his theory of how altruistic behaviour (e.g. female meerkats staying celibate to look after their nieces and nephews) can arise out of "selfish" genes, i.e. genes whose primary purpose is to seek to replicate into next generations. Essentially, it rests upon the fact that genes that occur in a particular animal is also likely to reside in their relatives, and this probability increases the closer the relationship.
My take is that something similar has ensured that homosexuality has survived to this day. Genes that causes an animal to be exclusivelyhomosexual would make no evolutionary sense since they would never reach the next generation but genes that causes an animal to be homosexual a certain X% of the time, or causes X% of the populationto be homosexual all the time (or some other combination in between) could survive and propagate, provided that this gives these genes a higher chance of survival into next generation. Especially for animals that live in close proximity with their relatives and/or in areas that face resource constraint, even if the gay animal doesn't take care of the offsprings of its relatives, a few animals giving up the chance for reproduction can greatly reduce the dangers of overpopulation and its resultant environment depletion in the locality of the species. Environment depletion due to overpopulationis seldom gradual - it usually occurs after being apparently stable for some time until pushed past the tipping point following which everything comes crashing down. (Thus the reason for concern by many environmentalists.) Thus, my theory is that the genes that predisposes some homosexual behaviour provides a pressure relief for overpopulation.
This would also work for genes that predisposes animals to asexuality, and this does occur in nature, e.g. celibate meerkats, worker termites (i.e. other than the queen), but it is probably easier to subvert a particular evolutionary development (in this case sex drive) than to de-evolve it. Besides, sex drive is still needed in general to propagate the species. Of course, there could be other benefits to the genes, e.g. increasing social bonds, gay animals who do take care of the children of their relatives, but to me, it seems that population control would be the primary reason. It appears that the prevalence of homosexuality is higher for social animals (i.e. animals that live in close proximity with each other), and also higher when animals are placed in caged settings (simulating limited environmental resources - how would the genes know that the animals they reside in are actually being artificially fed). For example, among the apes, homosexual behaviour is much less reported for gorillas, where each troup consists of a dominant male and his harem occupying a large territory with the other males roaming separately between unclaimed territories, as compared to bonobo chimpanzees, an extremely social species of ape.
