I see marriage as a partnership between two people. Each gets a lot out of the marriage - stability, a family, a home, security, companionship etc. So refraining from cheating on your partner is just one of the sacrifices you have to make in exchange for that.
It doesn't have to be a monogamous relationship to begin with. That's a societal construct designed to control people. It's only going behind their back if you have that kind of relationship to begin with.
If you can't trust a happy partner who's having their needs met, how can you trust an unhappy one who isn't?
A societal construct used to control people? What are you 15? I'd love to see the look on your face when you try and talk a girl you're interested in, into this.