Evoanth has posted The Evolution of Infertility in Humans suggesting that in evolutionary terms successful reproduction is more important than maximising the enjoyment of sex. I commented:
It clearly helps, in evolutionary terms, to have a loyal supportive male to help rear his children - so the priority must be pair bonding and not "sex is fun." Make sex too much fun for the male and he may put fun with new partners above loyalty - which means less support for his children - and more venereal disease. If all males do it all the time the overall effect is less stability for the children, more venereal disease and definitely no advantage for the female genes.
If a few males spread their oats widely there could be a statistical advantage to them - but if a female is in a more or less stable relation with a male who has just enough sexual capability to keep her pregnant he will still be the father of most of her children - even if a randy male gets in a poke or two.
Overall the balance could be that overall breeding success is highest with stable couples, supported by grandmothers, than , for example, a harem system where the male spends much of his energy defending his status rather than directly supporting his children - and where it pays any usurper to kill the harem's youngsters because he is not the father.
What I could have added is that team-work between males in small hunter-gather groups could also increase the survival chances of the group - and the genes of the individuals - and good team-work means trust - with minimum energy being expended in rivalry.