There is an interesting theory put forth by Bill Nye. According to Nye we need to evolve very quick to stay on the evolutionary chain. It's like a jogger where you have to keep moving to stay put. Now really small organisms like the bacteria and viruses evolve and mutate much quicker than we. To make our gene pool strong we must reproduce,
1) At a very fast rate and,
2) Mix up the gene pool
This means that we need to have sex as often as possible and with as many different people as possible. By doing this we will ensure that we are in step with the germs that give us tummy aches and other such ailments. All of this obviously has to be unprotected intercourse as reproduction is the main objective. What would the fidelity preachers, safe sex and AIDS campaigners have to say about this? Of course there's a lot to Nye's credit rather than just plain top of the head theories.