Cheaters have methods of convincing the people around them that their cheating was the fault of the other person for neglecting to meet their needs. Once they've done this, they can actually achieve a lot of leeway from the person they cheated on. For one, in a supposedly monogamous relationship, they've sent out a strong message that the betrayed person isn't enough for them alone, so if they still want to be with them, they'll have to accept either a non monogamous situation or live with the fact that their partner has to "settle for" someone who is inadequate. It's a good ploy, it makes you feel shit enough to put up with whatever they fling at you because after all, you're not good enough for them. They're doing you a favour.
Don't be one of those who allow cheaters to manipulate them in this fashion.