The rule is if your girlfriend wants to fuck another guy, you have to let her. You are not allowed to deny her the greater pleasure another man can give her.
I played junior hockey in the USHL and my girlfriend and I loved it when my roommate was joining us to fuck her. It was never a problem ever and she fucked him at least 2 times a week and she was on the pill so she was able to enjoy it all and I absolutely encouraged them to do it as often as possible and they did. I was in heaven