I've just sent a letter to New Scientist about sex, which they published:
Sir, Douglas Fox marvels at what he describes as one of the great mysteries of the human condition: why do humans spend so much time having sex without resultant pregnancy ("Gentle Persuasion", 9th February, 2002). The answer, it seems, is that it allows the woman's immune system to become accustomed to the man's proteins, thereby decreasing the chances of rejection of her foetus's proteins, when she eventually becomes pregnant.
If such reasoning is correct, it still fails to explain the great mystery of why this strategy is only adopted by humans (and a few other animals). If there really was such a clear-cut evolutionary advantage in frequent sex, wouldn't all animals be at it like rabbits (and, indeed, humans)?
Surely there is a far simpler reason why we humans have so much sex: because it's so enjoyable.