Monogamy and "never been cheated on" are separate things.
Anyway, I'm polygamous. I found myself ALWAYS cheating on my girlfriends at the time. It always broke us up, and I found I was never satisfied with a girlfriend. I became part of an open community, sexually speaking, where I discovered fetish and kink lifestyles (I'm not a perv, but I don't mind indulging people who are). I met one of my current girlfriends, introduced her to the scene (she was shy at first). We had a great time.
Then at work I started sleeping with a new girl, who is beautiful. I went out one night for a work do. My girlfriend turned up at the bar we were, and I introduced them. They hit it off. That night my girlfriend expressed physical desire for her, and I admitted to cheating on her. She got upset. She then went and got "even" with me by going and sleeping with this girl too (she'd originally gone over there to have a fight). I turned up to stop the "fight", and we all had an argument. It all ended in us being in a poly relationship, and I'm still not sure how that happened.
I've never been so happy and content in my life, and it really affects everything I do in my life (for the better). Our relationship is open, not to other people to join, but sexually it's open. Having a third person in the relationship helps loose a lot of that insecurity that my first girlfriend once had. Being part of the kink scene, there are a lot of play partners about, and so it's difficult to stay "faithful", by the standard definition, in those surroundings. I do not see any of this as being bad. FOR ME (although I'd argue for everybody), monogamous relationships are merely a societal norm, and an oppressive one at that. Our base instinct is to fuck, and fuck a variety of people. I cannot be satisfied with monogamous restraints, intellectually or physically.
I have two girlfriends I'm in love with, and who are in love with each other. The arrangement works out so well, I can't even explain it, you'd have to witness it. My Christian parents are supportive now that they've seen how we work, and all of my friends became acclimatised to it quite quickly, as did their friends. I don't see our situation changing any time soon, and while we have talked about children, we aren't sure how the emotional states will change. We are convinced that we can raise them normally, but mostly none of us feel that the time is right.
For me, life is good. I think it became good the moment it stopped being "cheating" and started being "life".
