Why isn't he mad about that, but is mad about this?

I think he didnt get mad at you for sleeping with somebody else......because he too might have slept with somebody else... and as for the nagging that is just a guy being a guy they hate to hear us nag all day long... or it could just be his way in trying to push you away... Maybe you should sit down and talk to him about it... And dont get mad at his honesty... Listen and try to understand what he is telling you... good luck