Why are humans so sexual?
Because our babies are born too early.
Yes, you read that correctly, humans have evolved to be highly sexual because our babies are born too early.
The reason our babies are born too early is because our brains are so big that babies have to be born before their heads grow too large to be able to get out of the birth canal. Which means they are born utterly useless.Unlike other mammals, whose young can generally stand and move around and get to teats to feed themselves, human newborns are incapable of even holding up their heads. And they stay highly dependent on adults until they are at least seven years old (and in the modern era it’s more like 17 years, sometimes 27 years or more!)
Other mammalian mothers can look after one or even a whole litter of infants on her own, because her young are sufficiently able to cope while she’s off hunting, or can follow her around reasonably well. Human young can’t. A human mother cannot look after an infant without support (at least, not in prehistoric days before we had single mother pensions). As humans evolved, both parents, and quite possibly the whole tribe, were needed to raise infants to an age of survival and usefulness. Sex evolved in humans to keep people bonded while they raised their slow-maturing children.
This fact also puts paid to the myth that men naturally want to ‘sow their seed’ widely. That is not a human reproductive strategy. Humans would never have evolved to the level we have if the men were off wandering around ‘sowing their seed’ and leaving the mothers to raise the kids alone.
Other mammals (with the exception of most other primates) only have sex to reproduce. They only have sex when the female is fertile, there’s no great pleasure involved, there is no purpose for it other than to reproduce.
Humans however, can have sex at any time, and it is highly pleasurable (or should be!) We can also continue to have great sex long after our reproductive years are over, probably because happy grandparents were needed to look after the children while the more physically able parents were busy doing other things like skinning hides, bringing in the harvest, and so forth.
Sex in humans has a social function, a bonding function.
Sex is so much more than just conceiving children; sex enables a couple to feel good about each other while raising those children. Possibly in prehistoric days people didn’t know about fathers, so everyone felt responsible for helping the women raise their children - and quite possibly they all had sex together (read ‘Sex At Dawn’, for a fascinating discussion on this possibility.)
So our big brains have forced us to evolve to have good sex.
Fortunately this big brain of ours has a dual function - not only has it necessitated us to have good sex, it has also equipped us to have good sex. Our enormous brain capacity allows us to think and plan and fantasize and be creative - all required for good sex!