Most people are polyamorous. They are just socialized into believing that they are supposed to be with one person for the rest of their lives. I think it is related to the number of D2 receptors in the brain. The more a person has, the more polyamorous they are; the fewer, the more monogomous. Same with wild animals, animals that mate for life have fewer D2 receptors and those that have many partners have more receptors.
I was married and never cheated, but I sure wanted too a few times. It's a natural thing. That's why I got into swinging. You can still have a great relationship... even better actually, and spice it up by bringing in others to satisfy those needs for variety, etc. Plus, the friends I've made in the lifestyle are some of the best friends I've ever made. I think sex brings a depth to friendship that you just can't get with platonic friends. You just have to be open and honest with each other and if you aren't totally secure in your relationship... don't do it. It's not going to FIX a relationship... just make it more better.
Also, research has shown that women cheat more than men... MORE than men most likely because it is easier for them. They like to find a guy that treats them well and has "resources". They will stay with them, but then sneak off with someone they find physically attractive. Men just like to have as many women as they can, it's in the genes of every male species.
Anyway, religion and society have tried to turn us into what we aren't. This is one reason we have so many psychological issues. My theories go a lot more indepth than this, they aren't pretty and most people don't like to think about them, but now is not the time.