Most Helpful Guy
The real answer? Brain chemistry coupled with some morality issues.
Some people are hardwired to stray. 43% of guys are hardwired to stay monogamous. The have no desire to stray, and their answers will all be "because they suck - I would NEVER do that". Which is half true. They wouldn't do it. But they literally do not have the brains to understand why someone would. So their answers regarding 'why' someone would do it aren't particularly helpful.
The study from the University of Oxford that provided the 43% of guys are monogamous (the numbers are 53% monogamous for women, if I remember) postulated that the 'stay or stray' brain chemistry was determined by the amount of testosterone received by the brain in utero.
Guys that tend to cheat are literally driven to cheat by their brains. It's why you will read stories about a guy with a beautiful wife who loves sex, and they will still cheat on her with a less attractive secretary or something. It is very hard to stop.
The reason morality comes into play is because cheating involves dishonesty. And a guy that is not able to do monogamy that doesn't explain that to his partner, and who doesn't try to have an honest, open relationship, or a poly relationship, or practice some kind of ethical non-monogamy is going to end up having to lie and break trust whenever he ends up in an 'exclusive'.
They cheat because their brains are wired that way, and it is cheating because they don't have a relationship structure that allows them to fulfill that need without lying.1