It's time for a little honesty and some things you don't want to hear that you won't like. Heeeeeeeeeeeeeere we go.
A guy in a long-term relationship will cheat on his girl and not break up with her because he doesn't want to lose her. He wants to sleep with other women that he has no intention of doing anything else with aside from boinking and then come back to you, the woman he actually loves. Now one might say, how can you truly love somebody and cheat on them. And you'd be right to ask that question. I think a lot of men have this uncanny ability to completely compartmentalize their emotional and physical needs.
Not that it's right. Be clear, I'm not saying it's right.
This dude can go sleep with a woman and have not one iota of emotional connection to her. She is merely fulfilling his desire to bed another woman or fulfil some physical need he's not getting at home.
Once again, that doesn't make it right or okay.
Whereas most women aren't able to separate the two (wouldn't sleep with a man for whom no emotional connection existed) man men are able to do so with ease. It's a sad but true facet of manhood. I actually know some women who have said they were able to do so as well. I can't say that I believed them but they brought it to me on their own after admittedly cheating on their boyfriends.
At the end of the day, your guy wants to be with you for the long-haul probably and has the wherewithall to realize that throwing that away by breaking up is a bad idea. Of course, he's also cheating on you which if brought to light would result in the same conclusion, so its generally a bad idea to cheat on your significant other for any reason.
He just won't realize how much of a mistake he might be making until its too late for him to fix it.