A. Because they can. Or, let me frame that differently. Because, for so long, they could. Without consequences. And because other men expected them to. And because women were encouraged to accept it, and were told that men’s infidelity isn’t personal; that it isn’t about them…..that men just had to, follow their organs wherever. Couldn’t control themselves. And because….oh, you know, because men controlled the universe, and if they wanted to have sex, they just did.
To this day, most cultures turn a blind eye to men who are sexually unfaithful (except those men who happen to be in politics), claiming raging hormones and the explanation that men are not, by their very nature, sexually exclusive, and that therefore it is unreasonable to expect them to be. But we know that it is as much about entitlement as biology, and in many cultures men can have more than one wife; in others, they can have mistresses. These allowances are built into the system, the institution, of marriage. (Note, though, the double standard: There is no culture on your planet that allows women the same sexual freedom. Women, it is taught, even today, are characterology lifelong monogamous creatures. ) I have argued with many a man over this point-and believe me, it is futile. these are men who really believe that a woman isn’t a “real” woman unless she is incapable of straying-while a man isn’t a “real” man unless he strays pretty much all the time.
Things have changed. Not only do many men want to be and are sexually faithful, but most of us expect both genders to behave. (By the way, I hate the term “cheating”–I use it here because that’s the word so often used when I’m asked the question.) The double standard has largely fallen away in American culture, both in fact and in philosophy. So wives are no longer asked, as they once were, to turn the other cheek when their husbands have affairs. Moreover, it comes as no surprise to anyone these days that some wives, like some husbands, have affairs.
In American culture, the bigger question is no longer, “Why do men cheat?” but “Why do men AND women cheat?” Because if you look at the statistics, and I do, you know that, unless we’re talking about homosexuality, an affair takes a man and a woman. It makes no sense, then, to think that only men are “cheating,” right? After all, with whom do we think all these guys are sleeping?.
Stay tuned. 
We’ll be talking about this a lot. Your goddess knows a great deal about it, both as it occurs on earth and in the heavens!