Most Helpful Guy
I haven't read the others, but I can already imagine how much you are getting "It's our evolutionary pressure to spread our genes" and stuff like that.
That, of course, assumes sex is all about reproduction, but humans went beyond that a long time ago. In our closest ancestors, the chimps and bonobos, sex is how close bonds are established. In chimp societies, the premise is mostly violent: To sexually starve the "lesser males" until they go insane and either beat up and dethrone the "alpha", or rape a female and thus have the alpha kill him. That's why I hate "alpha males".
For Bonobos, violence isn't used to get sex, but sex is used to avoid violence. It keeps them closely bonded to each other. Bonobos are one of the least violent creatures we're related to, and also one of our closest relatives.
And it's important to note that in almost every highly-intelligent, highly-social creature, sex is recreational and used to establish complex social bonds. They evolved past sex as a mere means of reproduction, and made it primarily recreational with the side-effect of producing offspring.
Okay, that aside...
Here's a bit of something to start with: link
Men's locus of identity is almost completely external. That is, a lot of what reassures a man that he's "good enough" comes from things like attention from women. We're usually not told we're good-looking or attractive, and are complimented quite a bit less than women are, so we have little idea of where we actually stand.
So for some guys, they might cheat because deep down, one girl liking them doesn't make them feel desirable enough. It's kinda like asking your mom how you look. All the compliments in the world, and they would always be taken with a grain of salt unless you get an opinion from others.
Another that could be likely is this: You are is most important emotional connection, but your libido is much lower than his, so he looks elsewhere to keep himself sexual satisfied without demanding you have sex more than you feel comfortable.
The vast majority of cheating comes from mismatched or uncommunicated libidos.1