From a biological perspective, the primary reason for anyone to have sex is to procreate. Sex does have some additional value in social bonding but if procreation wasn't driving the show there would be zero sex. Your own or anyone else's interest in sex is a structural expression of biological requirement for procreation. Of course, this isn't the feeling or narrative interpretation we put on sex but why would it be? We are built by evolution not to understand ourselves and the world but to have capabilities and impulses that work - or, more accurately worked on the plains of Africa 100.000 years ago.
The biological value proposition of sex for males is quite different for males and females. At the minimum inputs, a male contributes a teaspoon of semen to "produce" a child while a female (human) contributes contributes 10+ years of physiological expenditure: gestation, birth, lactation, care, food sharing, protection, etc. This is actually more or less the way things work in chimpanzee society. The males spend a lot of time fighting over the females in order to have sex and the females look after the kids. A female chimp won't let her child out of her sight for it's first six years or so; apart from external predators, other chimp mothers may treat it as a protein supply for their kids. She is the quintessential obsessive mother.
How far have we evolved from this chimp model? Obviously, co-parenting is adaptive or we wouldn't have taken it up. On the other hand, the (new) cost of co-parenting falls mainly on the males so the females had to make it attractive to males. How? The obvious candidate here is the male obsession with sex. Human females have evolved continuous sexual receptivity and hidden fecundity - features which are AFAIK otherwise unknown in megafauna. It's a pretty good guess that this evolved to facilitate a deal: coparenting for sex. To augment the deal, we can expect layers of domestication. As the dog that protects the family but doesn't bite the kids was selected, so women selected men who demonstrated personalities compatible with reliable parenting, and in the other direction, men may have selected fidelity and enhanced sexuality. The permutations of these qualities, and the ensuing cultural constructions provide serious entertainment for us, both as observers and participants.
Like all deals, the prospects for cheating and backsliding are ever present: men will skip the housework or the home and women will skip the sex. I'm not proposing any particular solution to this but I do recommend that we acknowledge the basic biological deal, and it's problematical nature. From an evolutionary perspective men are newcomers to childrearing as women are newcomers to recreational sex. It's a deal that kinda works but it's going to require our ongoing efforts to make it work well. If Evolution has Intent, it surely isn't to ensure that every man gets his desired frequency of sex or that women get their toilets cleaned at their preferred frequency. This may require some conscious effort and even a little guile.
(Originally posted as a comment on this article
http://wheelercentre.com/dailies/post/d6d50e247f8b/)
