Given the variety and abundance of studies, along with my reading on evolutionary biology, human biological history, and sexuality in general, I think it's a simple matter of 'we are built to be profoundly sexual creatures.' Just look at the facts of orgasm and I personally think it becomes clear that it has to be a critical bodily function. If you refuse to do anything to elicit one intentionally, your body will eventually take over and force one to occur in your sleep. That is not something the body does generally with other things except critical ones. There is certainly an argument that perhaps the purpose is to improve sperm quality in males, but females have nocturnal orgasms as well (though the mess after puberty makes it quite a bit easier for males to recognize it has happened).
We could have evolved to have a 'mating season' as almost all animals have. We could have evolved for females to give visible indication of fertility. We could have saved "wasting" the positively monumental amount of energy we pour into pursuing sex. We could have evolved so that sex was not even pleasurable but driven by instinct. We didn't have to evolve women going through menopause (and living a substantial amount of time afterward). Almost every single facet of human sexuality, if viewed through the lens of "sex is for reproduction", looks nonsensical if not evolutionarily suicidal. But through the lens of "sex is primarily for pleasure and social bonding," every bit of it makes sense. It explains why we stuck together in groups, the only thing that let weak, slow, stupid chimps without fangs, claws, venom, natural armor, or other similar natural advantages (except maybe endurance) survive long enough to have our social environment provide pressure for brain development and development of speech. We stuck together to preserve access to sexual partners, because it was pleasurable, and it made us the most powerful species on the planet. It makes sense that those who 'needed' it more were the ones to pass on their genes the most often.