Elders are an important information vector in human societies, and a critical one in preliterate ones; they have a long memory, an uncommon education, and a generally high level of respect, time for reflection, and external support. One of the major watersheds between Neanderthal and Cro-Magnon was the far more efficient care for the elderly among the latter. More members of your group living to old age meant that you would never live in a society without generations of learned experience. While all other things being equal women tend to live longer, in early man all other things weren’t equal - women had to expend enormous amounts of resources to go through pregnancy and childbirth, and the unprecedented likelihood of death during childbirth made it a risky endeavor.

Menopause was helpful here - it meant that women’s reproductive lives would end past their ability to survive the average pregnancy, costing little in reproductive terms and being of tremendous benefit to society. But there would have been a tremendous social advantage to the existence of spinsters - infertile or minimally-fertile women - in that they would be much more likely to live to old age than their fertile sisters.

While same-sex sexual contact is common among primates and other animals, homosexual pair-bonding is only frequently observed in adulterous species - those with marital bonds often broken by their participants. It probably serves a significant social purpose in decreasing the mating pool without decreasng the number of socially active adults or causing sexual tension; whatever its purpose in reproductive biology, the prevalence of homosexuality in human societies would have a desirable side-effect of producing spinsters, who would significantly boost the average number of elders in a group at any time. In purely natural-selective terms, being homosexual means your relatives are more likely to pass on their genes.

The result of more elders is an increased reproductive incentive towards intelligible speech, mechanical deftness, and innovation, and away from strong instincts. In social terms, it means non-elders live longer and better, have more time to think and participate in culture, and have more information to pass on to their children.

It might be an exaggeration to say that the existence of (near-)exclusive homosexuality is responsible for human civilization, but it might not be. And at the very least, it’s pretty important.