...supposedly there is an evolutionary 'survival-of-the-fittest' theory for the differences in men and women in regards to fidelity and monogamy (and selectivity). It has to do with reproduction and goes like this:

Assuming a healthy male, with each ejaculation men have the abilty to fertilize an egg. Their role in these matters is merely a passive one: to provide fertilization - to any egg, anywhere, anytime.

On the other hand, women have a window of only a few days each month within which to conceive.

Therefore, women tend to be more selective in choosing fathers for their children. Ostensibly, the mother would want to choose the 'strongest' male to fertilize her eggs in order to maximize the probability that her children survive.

It could explain why some men have a problem keeping their pants zipped and why some women play "hard to get."