"3. The female genitals play a deciding role in allowing paternity. That is, the vagina is not an equal opportunity sperm acceptor but picks and chooses the best man (or sperm) for the job. […]
4. Because of this fact, nature loves variety - particularly sexual variety. That’s why in most species of life monogamy is rare. It is not unusual in the animal kingdom for a female to mate with multiple males during her breeding season. That way she gets to sample a variety of sperm and pick the one that is genetically the most compatible with her body. It is mother nature’s way of ensuring healthy offspring."
12 Interesting Facts About Female Sexuality