Christmas -
Actually while I don't have a reference to hand, I'm pretty sure I remember seeing studies that women ARE more likely to be promiscuous or unfaithful during the most fertile part of their cycle.
Female promiscuity is of course not unheard of (just as women who like porn are not unheard of). But the point is that women, evolutionarily speaking, are massively restricted in the number of offspring they can have. Each requires 9 months' pregnancy, years of breastfeeding as a drain on her time and energy, and the very real risk of death during childbirth. Remember too that for most of our evolutionary history, women would have spent most of their entire adult lives either pregnant or nursing. A woman who is pregnant with a man's child has no evolutionary incentive to shag other men, because she can't get pregnant with their children at the same time. What she CAN do is try and make sure the guy will stick around to maximise the chances of her offspring surviving, through all the means of pair-bonding such as sexual fidelity.
A man, by contrast, can get her pregnant and then go and get a whole load more women pregnant before the sprog even arrives. Sure, it MAY be in his best interest to stick around and ensure that one of them thrives in a strong family. OTOH it may make more sense to just dip the finger in as many pies as possible, so to speak, and trust that at least some are more likely to turn out well that way.
Even better, he can navigate a half-way path. Stick around and support the kids, participating in the pair-bonding and monogamy if only because it keeps the woman faithful (which is necessary to avoid being tricked into investing in kids who are not his own) - and have the odd dabble on the side as well. Insurance policy, if you like.
Hence the happily married man who still eyes up women on the street and looks at porn. It's not going away any time soon.