I can see how parents who love their daughters and want to protect them from the life-changing consequences of early pregnancy may come to think abstinence is the only way to achieve that.
True, although over the past few months we have been treated to a high profile example of how well that works.
Interesting post, lia! You are right that heterosexual couplings are fraught with complications. In fact, when I see a straight couple in a movie immediately have sex on their first night together, I'm always somewhat bothered. Not in a judgmental sense, but I always wonder, do people still do that very often in real life? Do they live happily ever after? I do know a couple of two who had sex immediately upon meeting and went on to have good relationships, but I know of countless more situations where the sex was the end of it.
Men who split after a one-night stand have at least an evolutionary basis, if not a moral one, for doing so. Since men can have virtually an infinite number of children, it makes sense to spread their seed as widely as possible -- the more offspring they have, the more their genes will carry on (some of them presumably in boys who grow up to echo those promiscuous habits).
But also, if a woman has sex with a man on their first night together, it may imply this is a frequent habit of hers. So if she's having sex with a lot of men, and then gets pregnant, which man is going to be stuck providing the time and resources to raise a child that may not be his? There's no evolutionary payoff in that -- his efforts do not go to passing his own genes along. Therefore, again from a purely Darwinian perspective, it makes sense for the man to move on quickly after a brief encounter.
Evolutionary psychology can suck sometimes.
It's funny that movies have no problem breaking this evolutionary "law," (maybe just for the sake of succinct storytelling), but rarely break the "law" that says males are rarely younger than their female partners.