A relationship is never just bad or just good experiences. Your boyfriend and his ex also were together in "crime" cheating on S and presumably "enjoying" it or one or the other would not have done it. They have a history that we cannot know because we were not part of it. Relationships that "end" well still often have bad/rough spots? People cheat and their significant others forgive them and they work through the betrayal together and get to a different spot in their relationship. Love is not completely rational where one can say, if someone cheats, then the other must feel X/cannot continue to love. There are always more feelings and interactions going on than just the obvious ones. That you refer to his ex as "abusing" him I think speaks more of your feelings after his telling you his experiences than about his feelings of his actual experiences? He does not necessarily feel "abused" because he was cheated on or the degree to which he feels something does not necessarily match what we might feel in that situation.

It is easy for me to imagine how I would feel in a situation and then assume other people feel the same way I do, whether or not I have ever been in that sort of situation. I remember when I was young and swore I would never date a married man and dated two, my husband being the second. I try to keep away now from deciding what I would/would not do in a situation I am not currently in or maybe have never been in. Being in an experience and then seeing how I do in fact respond is the actual, the reality of the experience, not my thinking about it.