For me personally there is no exception. When I am in love with someone I give it my all. If for some reason that was not enough to cause the person to remain loyal the first time then there is nothing I have left to offer differently a second time.
The only way I could ever imagine forgiving someone for cheating would be if I felt I played some role in pushing them away, (neglect, distant, verbal/physical abuse, inconsiderate, unsupportive, unromantic, lack of passion, unaffectionate....etc) Any of these actions makes it easy for a stranger to put a smile on your mate's face and in their heart. Ultimately cheating a personal decision one makes just as forgiveness. Each of us is entitled to have our own "deal breakers".