Monogamy is an unnatural state for humans to be in. At the deepest level, many men want to have many mates. Just like at the deepest level, many women desire a stronger man, a "provider" so to speak. Before you try to figure out why men cheat, first you need to figure out why it is that serial monogamy is important. People have many friends, all who generally bring different things to the friendship. The same way I have friends who are musicians and friends who are artists or athletes or political can be applied to sex in relationships. Don't think about it as them getting bored, think about it as them wanting variety. Really though, the reason why men, and women, cheat, is because their selfishness wins out over their selflessness. Will power wanes, and they do what they want to do despite potentially hurting another person. All of that being said, though my mind has wandered from time to time, I've never cheated on anyone, so I'm not trying to justify this to make myself feel better. And, I've never been cheated on, because I can't really be cheated on, in that I don't mind if my partner strays. As far as I know that never happened, but... I never could figure out why people are inherently jealous. It's a foreign concept to me. Anyhow, there are likely many men out there who never have and never will cheat. Or there are none, who knows.