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That's a topic considered by evolutionary psychology...
One possible explanation is evolutionary motivations centering around fertility.
The fact that it's outwardly very difficult to tell if a woman is fertile makes it (evolutionarily speaking) advantageous to the male to get semen into her as often as possible. The possibility that she might be having sex also makes it imperative that he inseminate her often, so as to increase the proportion of his sperm inside her, increasing the likelihood that if she IS having sex with other men then any child she conceives will still be his, and carry his genes.
Now, the first obviously applies to the female as well, EXCEPT that since her behavior MAY be influenced by the same hormones which ebb and flow with her periods of fertility, she might achieve the same likelihood of conception while being, on average, less likely to initiate / be receptive to sex. (because she does so more at certain times)
In fact, some research has indeed indicated that women are more likely to initiate sex when they are ovulating.
The second reason seems to be valid only for men, because while a man can increase the likelihood that any child conceived by an unfaithful or non-monogamous partner will result in an offspring that is his by having sex more frequently, a woman cannot increase the likelihood that any child she conceives with an unfaithful or non-monogamous partner will be genetically hers; that probability is fixed at 100%.
Now, this all applies only to the effect that evolutionary imperatives have on our sexuality. There is a bit more to it than that, of course, and a woman may initiate sex or be sexually receptive with her partner simply because she enjoys it and/or wants to please her partner. Those two motivations are both, of course, highly variable from woman to woman, and so they may account for some of the variation that exists between the sexual habits of one person and another.
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