I think men cheat because all men are designed for adventure, be be strong, courageous, passionate. It's part of masculinity, it's some of the very identity of masculinity.
Masculinity can only be passed on from man to man (the ideal would be a good father teaching his son that he is good enough, strong enough, smart enough, etc.). Masculinity, manhood, is bestowed from a father-figure to a boy, especially from age 9-18 (give or take). If men never "receive" their masculinity, if no man ever teaches them, encourages them that they are good enough, strong enough, etc., he will look to women to give him that assurance.
This also helps reveal the truth that whole [meaning healthy, normal] men get more satisfaction out of pleasing their lover than they do out of the physical pleasure--they want to know they can do it, they are good at it, can take care/provide, be good and strong. Think about it. It doesn't matter how good the sex was for the guy, if his lover was unsatisfied, he will feel inadequate, impotent.
When a woman fails to give a man his masculinity, which she will, without exception because she is not a man, he will look elsewhere. If a man has never really felt like a man he either (1)retreats to a sensitive, shy, gentle (sometimes gay, even?) man or (2)be obnoxiously masculine and completely over does it with his attitude, his clothing, talk, walk, belongings, trying to show everyone he's a man. A whole (balanced) man who knows his masculinity will be a balance of both, and it will be life-giving to his friends, family, lover.
When men are not men, they fail women because women want men-men, those who know their masculinity and are comfortable being both passionate, gentle, strong, quite, adventerous and faithful.
When women are failed by men, and also when women fail men because her own heart is hurt and she doesn't even recognize her own beauty (either by trying to bury it or show it off so the whole world can see it), the man will go elsewhere to look for adventure.
It's all very tragic and messy, and it doesn't have to be like that. I believe people were designed for committed companionship. How can you say you love someone when you can't stay with them long enough to learn how to communicate with them, even the deep parts they are afraid to show? When you can't stay with someone long enough to know all the ways to give them pleasure (physical and otherwise) and explore new ways.
"window shopping" for love, attention, sex, leaves you wanting and unfulfilled. Even if you're high on the excitement of adventure for so long you hardly notice you're not satisfied. One day it will hit you that you have no one committed to you and you are committed to nothing. You are detatched, empty, lonely and bored, without adventure. But it doesn't have to be like that.
There is an *awesome* book on all of this called "wild at heart". It's specifically written to men but also accommodates and *really* speaks to women about femininity and how the two fit together, work togther, are different.