The simple answer is that there is an imbalance of power when it comes to the sexes. Historically speaking, men are the go-getters; they go out and win the bread and whisk the women away. This puts them on the offense of sexual competition. They are the ones to always initiate everything. Women, on the other hand, stay at home and take care of the kids or gather berries or whatever. This puts them on the defense of sexual competition. The men come to THEM, not the other way around. Thus there is a type of aggressive/submissive hierarchy going on.
Biologically speaking, it is more advantageous for a man to sleep with as many women as he can, whereas it is more advantageous for a woman to sleep with the most qualified man in terms of genetic superiority. This is because of the gestation period in women, which basically says that men can fuck five women in a night, but women can only be pregnant once every nine months.
So already we have an argument for the gender-role fulfillment theory, which states that men or women who deviate from the time-tested roles assigned to them are considered abnormal. Men who aren't living to their potentials are considered weak by the stronger men, and those women are considered immoral by the sexually-ascetic women.
This is how a social biologist would explain this phenomenon. However this explanation implies that we think on a much more biological level than we actually do. It is true that men still think of lesser men as "weak" in the strictly biosexual sense (and of course the comic or feminist would say that this is because men are more primitive and only think of one thing), but morality is not a biological phenomenon, thus women are not fully explained. We must go deeper.
If women and sex is a moral issue, then where do we go to learn about the origin of morals? The church, of course.
Because men are considered the "go-getters" and women are more passive, there is created a sort of game, a game in which the men are the players and the women "play hard to get". A woman by definition is expected to be discriminating when choosing a partner; she is not apt to just let anyone in. But men are supposed to be very lax when accepting a partner. Thus depending on who takes the initiative, the women is hard to get and the man is easy.
Our moral system usually states that taking advantage of an unfair situation is wrong. Say that women wanted sex as much and as often as men. Then there is absolutely nothing to worry about, because most men would jump into the arms of an hot, initiating woman if given the chance. Thus this is an unfair situation that one can take advantage of. So then, women can't fuck men as often because it would be taking advantage of an unfair situation and lead to such vices as greed and gluttony.
So there you have it. I hope you will continue to come back later for more of Cone's Super-duper Lectures on Stuff That Happens.