Personally I really would never cheat. I've never been in a relationship and I'm scared to be cheated on if I am ever in one(that's not the reason why I haven't had relationships though).

My belief is that a successful relationship doesn't have to last a lifetime in order to be considered successful and fulfilling. I think most relationships aren't going to last that long, as in the feelings won't, and that is absolutely ok. For me, a successful relationship is one that is fulfilling while we both want and care for the other and that is respectful and positive while those feelings exists for both of us. Thus, it's ok if one falls out of love or isn't satisfied anymore, it doesn't mean the relationship was a failure or a waste.

So cheating is out of the question, I don't believe in staying with someone and cheating behind their backs to "spare them the suffering of leaving them" or not "fail" in the relationship. If you're in a monogamous relationships then once you feel you can no longer be fulfilled by being with that person and no one else on the side, you should take it as a cue that the relationship has run its course and it's better to be honest about it and finish it before you sleep with anyone else.