not sure what to do about my BF

he and his friends do things that I don't approve of and its quickly becoming more often. I'd rather not say what so please don't ask. but it's really an issue for me. I'd give him an ultimatum but I'm afraid he would choose the activity over me. because of my disapproval, he thinks I don't appreciate anything that he does for me. I do though. and I love him very much and I know he loves me. but his activity is putting a wedge in our relationship. has anyone else ever been in a relationship where you disapproved of something your partner was doing? I need your advice! I don't want to leave him but I also don't know if I can deal with things....