This is a problem because the person that cheated is accepting that they deserve to be treated like shit. It is understandable feel angry if the person cheated on you, but once you forgive that person you need to stick to your word. Also, if the person that cheated promises not to again, they should stick to theirs. Forgiveness doesn’t mean making the other person feel bad. The goal is to be able to laugh at it someday, but you should never accept being treated badly in a relationship.