I think a lot of it depends on the extent of the cheating. One drunken slip-up is a lot different than, say, conducting an affair for months on end or cheating with a new partner every weekend or something more consistent like that. The latter type is much more of an indicator of something being wrong with the relationship, and, in my opinion, would take much more work on forgiveness and functionality in the post-cheating relationship.
In the event that a single, isolated incident of cheating occurs, I think the party that has been cheated on needs to look at things rationally. Yes, the cheater did something wrong, but considering other factors should be done as well. For example, was the partner drunk at the time? How extensive was the physical interaction? Is it as big a deal if someone just kisses someone else, or does it only become a big issue when clothes come off, or when sex is involved? How intentional was the act? How quickly did the cheater confess, or did he or she intend on never telling? Was the person the cheater had relations with someone they know or someone random? Regardless of the answer to any of these questions, a lot of talking needs to happen, all of it completely honest from both sides. While I believe that the party that has been cheated on is completely justified in being upset, it is also necessary for them to put things in context and try to understand the whole situation rather than spend the entire time being angry and stubborn.
If an involved affair has happened, that’s an entirely different story. Chances are, there is something that the cheater is looking for in an affair that they aren’t able to find in the relationship, so, above all, if the couple wants to repair the relationship, they have to address the shortcoming(s) that led the cheater astray. The cheater must admit fault and guilt, however, and the partner shouldn’t use the situation as a fall back guilt trip whenever a problem arises. At some point, they will have to address it and find a way to deal with that as its own problem. After the proper apology and display of remorse and ample healing time, I don’t think it’s fair to continue to bring it up and use it as a trump card in any and all arguments.
Really, regardless of the situation, communication is the jumping off point. Both parties must also be willing to salvage the relationship as well as move on, otherwise it’s just futile.