Because women see sex as a mental, emotional bonding thing that means forever
after and all that.
Men on the other hand see it as sex, period. Feels good so do it kind of thing. Now that is not to say that men are cheaters, just that their take on sex is different than ours. If he sees a naked chick, he is gonna look whether he loves you or not. If you ask him if he looked he will deny till the cows come home, cause he dont want to fight, but yeah, they all do.