Hey, no one's yet mentioned the good old cuckolding fetish. Some people really get off on having their partner cheat on them, and then come back and tell them every dirty detail. It's not even a particularly rare one.
Check out this article and see if it hits a nerve:
http://www.thestranger.com/seattle/S...ove?oid=115476
I'm not saying this is definitely you, and of course if the relationship is poisonous then get out...but your first post made it sound like you actually think it would be hot to have him dick around and then come back to you.