Once a cheater always one?

this is tough, my boyfriend cheated on his ex to be with me but she was also cheating on him ( I know that doesn’t make it better ) . Everyone tells me he’s gonna cheat on me because he cheated on her. He doesn’t even show interest in talking to other girls he doesn’t even speak to any at all like I’m the only girl in his family except family because he has a landscaping job with all guys. Every time I tell him my friends say once a cheater always a cheater he says oh so If I kill someone once I’m going to do it again? I don’t think he agrees with that statement and offends him. What do you guys think? I get worried at times but we been together almost two years.