three some..

So my boyfriend is out of state and we were talking on the phone and he randomly says " we should have a three some". And I was like wtf why? (He's talked about this in the past and I said no) and he was like well it's on my bucket list before I die. Mind you he's 18 🙄 and I'm 19 almost 20. We've been together for almost 2 years now and he should know I don't want to have one because I don't want him having sex with another chick in front of me! I'd punch her in the face and tell her to leave. I wouldn't want him having sex with another chick behind my back either. But I said so you wanna have sex with another girl? And he said well yeah isn't that the idea? And he then said, we'll tag team and I was like wtf. Like no I don't want you to do anything with her and I don't want to do anything with her because I love you and want only you. So I don't understand why he wants one so bad. It makes me nervous and scared that he might leave me..and if we had a three some, what if he wants to have sex with her? Like I'd be fine with making out with her and stuff. But not having sex with her and I would not be okay with her having sex with him at all!! Lady's do you understand? Or has anyone been in a three some? Tell me your stories and your opinions please.