...which is why I'm such a slut.

Scientists have done a study on pigs (whose reproductive systems are very similar to that of humans) that provides:

possible biochemical proof that the reproductive system of female mammals can "sense" the presence of sperm and react to it by changing the uterine environment.

And evidence for:

a detection and selection system that alerts females to the presence of different kinds of sperm and then triggers mechanisms in the oviducts that control sperm transport, binding and activation for fertilization.

So what does that translate to, in the natural world?

"We know sperm selection exists in nature, especially in promiscuous species, when females mate with several males," Fazeli said. "Baboons are a good example. During one reproductive cycle, if the female mates with several males, most of the time the offspring belong to one of the males -- not a spread between all of them... We are now seeing what can be the molecular basis for this effect."

So let me just re-capture the awesomeness for you, in case you missed it. This study suggests that after sex, my vagina observes what kind of sperm she got, and then she decides whether she wants to make it easier or harder for that sperm to fertilize her eggs by altering the uterine environment. We are well-aware that women select mates based on their fitness, by evaluating things like strength, status, facial symmetry and pheremone content. What this study suggests is that after a woman has sex with a potential mate, her body goes through another evaluation process based upon sperm quality. And that, pure and simple, means that it is biologically advantageous for women to be promiscuous. BAM.