I am missing your point. Enjoyment is a side effect of sex, not its natural purpose. I still don't see where you're going here. Why do we have that side effect? Sex is fun because if it wasn't, nobody in their right mind would do it. There is a cost to acquiring a partner (not necessarily monetary value, mostly all the invested time and emotion... the actual biological cost in humans is buried under culture and etiquette), you can also get a venereal disease, your genes only have a 50% probability of being passed on, it leaves the door open for 'jumping genes' which muddle up our genome, and over time systematically demolishes the all the best gene combinations (someone's hatchet example). Biologists can sometimes come up with tantalizing theories on how sex replaced cloning in eukaryotes but at the face of it, we still have no idea.