I just can't wrap my mind around cheating. I don't understand the motivations. If you want multiple partners, then state that when you first start dating someone. Why be greedy and play the part of an exclusive boyfriend or girlfriend when there are plenty of people out there who have no problem being non-exclusive?
Or, just break up with the person. Sure a relationship gets comfortable and becomes difficult to break off, but I'd have a hell of a lot more respect for someone who said to me, "I don't love you anymore and I want to see other people," instead of someone who cheated on me. It'd hurt at the time, but in the long run, I'd be much happier.
And no matter what anyone says, someone who cheats on you does not love you.