The capability of "being social" is so important that crows play social games, and their brains are not much bigger than an eyedropper. Homosexuality and polygamy are present in all the species where being social is not just a state machine orchestrated by pheromones (i.e., ants). It is as if Mother Nature would use all the tools in its arsenal to clump together individuals with enough shared interests, and hijacking the mechanisms of sexual pleasure for that goal is a hell of a temptation for our Mother.

In humans, the one that brings the most joy to the collective is likely to enjoy others' presence and company, even in non-orthodox says. Being social will afford that person survival when somebody more ill-tempered may fall off a ravine in the middle of the winter, not to be found until the next thaw. And there was an awfully long age when all hominids had to guide them were their genes and instincts.

In terms of evolution time, we invented two seconds ago systems of belief and moral codices. They made us so successful that there wasn't enough land anymore to sustain us. Rather than optimizing for individual survival, we started optimizing for the survival of our groups and the systems of beliefs that clumped them. And for their armies.

If you live in a place which is mostly deserts, with scarce water and productive land, it would make sense to keep your population numbers low. Unless you are planning to invade your neighbor for their land, or you are expecting your neighbor to invade you at any moment. In that situation, you have little choice but to be the one with the biggest army and the most sexually frustrated, violent males ready to perish in a blaze of glory. Sodom is just a little off from the Dead Sea, and our crops don't like to grow together with salt statues.