Boyfriend Breakup sex

Ok, so my boyfriend broke up with me about a month ago . We decided to get back together because we both love each other and can't keep away from each other. He said he doesn't want to keep any secrets from me and told me that he had a casual sex with some girl I knew(not a friend). I'm really hurt but I know I shouldn't be as much hurt as I am because we were broken up . Any advice on this? Please someone?