I had a similar thing happen with me. Well, not exactly the same. I think he's just doing it to make you feel guilty and to kind of have the upper hand if you get me. It's really childish, and you need to make it clear to him that although it was wrong of you to do that behind his back, it was no reason for dumping you, [which he's obviously realised by getting back together with you] and that if he can't accept it and let it go then you may as well not be together. He can't keep dumping you when he feels like it, then saying oh yeah I was being an idiot again let's get back together. That's not fair on you. Sit him down and talk to him about it, you can't be expected to put up with him reminding you of that forever. If he loves you and wants to be with you he'll agree to put it behind you both, even if it bothers him.