Sexually Adventurous

My boyfriend is very sexually adventurous. His biggest kink is me having sex with other guys. I am not very comfortable at all with this, I like to sexy other guys in a group chat... but sex is different. I'm afraid to say anything because before we were a thing, I had sex with another man. My now BF had assumed we were an I team and it started a fight. I'm afraid that he will bring it up and I don't want to do that. I would never cheat. I didn't know we were a thing and he knows that. It's just complicated to be open about sex with him sometimes...