I might get more hits with a title like that.
Prot (Kevin Spacey) described sex on his planet K-Pax as an indescribably painful experience. Like a kick in the nuts. And that really hurts you know. Well, it got me to thinking, maybe that's how ours will evolve into too.
Sex is our method of reproduction. Reproduction is a natural instinct of any living thing, a primal urge to propagate, and multiply. That's why sex is so pleasurable. We feel all those incredible, mind-blowing sensations in order to encourage reproduction. (Not that I would personally know, of course, being chaste and pure.)
But then in later millennia things will change. Earth will eventually be overran with humans. We'll hopefully be able to develop technologies that will prolong our lifespan. (If we manage not to blow ourselves up beforehand.) Looking at the world now, both these extrapolations on population and longevity are highly probable. A planet jam-packed with long-lived people is certainly a threat, one that will force our race to adapt, to survive. I guess what I'm saying is, there will come a point when we would have multiplied too much for our own good, and that evolution will then kick in and do what it does best.
If reproduction hurt like hell then we won't be jumping each other's bones every free moment, now would we? Voila, problem solved.
Anywaaay, there's always stuff like contraceptives, vasectomy, castration, biological weapons, feminism, etc., so this might all be moot.
But maybe S&M is an omen.