Cheating, like so many bad things in life, is a cycle. Whatever you've suffered, it seems to be human nature to then go on and inflict that suffering on others, and cheating is no different. They've hurt you, so how do you pay them back? By cheating in return. It's not just that they want to get back at you, though. It's also some kind of need to make people feel what they felt.
Without point fingers and playing the blame game, it's a high chance that cheating on someone will produce the exact same results. Have you put them through hell? Then they're going to do it right back, no matter how much they say they've forgiven you. If you truly want to repair your relationship, then you're going to have to be on guard - and be prepared to accept - the inevitable retribution.