Natasha Mitchell seems to have hit the obvious answer. If women didn't enjoy sex and all mating was reluctant or forced:
And the female orgasm isn't a remarkably complex adaptation, either; the genetic material responsible for the male orgasm was already present and relatively easy to repurpose. (Assuming that the male orgasm came first instead of the reverse, though simultaneous development is more likely than either.)
Do you think that people only have sex because they might have an orgasm? Really?
>Elisabeth Lloyd: I don’t actually know. I think that it’s at a very problematic intersection of topics. I mean, you’re taking the intersection of human evolution, women, sexuality – once you take that intersection you’re bound to kind of get a disaster. More than that, when evolutionists have looked at this topic, I think that they’ve had quite a few items on their agenda, including telling the story about human origins that bolsters up the family, monogamy, a certain view of female sexuality that’s complimentary to a certain view of male sexuality. And all of those items have been on their agenda and it’s quite visible in their explanations.
>Natasha Mitchell: I guess it’s perplexed people partly, too, because women don’t need an orgasm to become pregnant, and so the question is: well, what’s its purpose? Well, is its purpose to give us pleasure so that we have sex, so that we can become pregnant, according to the classic evolutionary theories?
>Elisabeth Lloyd: The problem is even worse than it appears at first because not only is orgasm not necessary on the female side to become pregnant, there isn’t even any evidence that orgasm makes any difference at all to fertility, or pregnancy rate, or reproductive success. It seems intuitive that a female orgasm would motivate females to engage in intercourse which would naturally lead to more pregnancies or help with bonding or something like that, but the evidence simply doesn’t back that up.
The whole discussion. It backs my theory that using evolution to explain current traits seriously tempts people to make things up.