Help- hubby wants threesome

I know that this is discussed a lot in here but I kinda feel this is a bit different post. What I need help with is more how to say no. It is a fantasy that we both have to be with another woman at the same time BUT beyond talks and occasionally looking at apps together it’s never gone any further. Recently he was looking at apps without me and has gone as far as talking to a friend in that lifestyle and she introduced him to a guy that could help “us” (more like him) find a third to participate.

I can’t go down this road, he has had 2 emotional affairs, one VERY recently. We have been married 15 years and I am 8 years older than he is. Our sex life is better than it ever had been, but I am afraid if we go this route he will like the other girl better and may ultimately cheat or out right leave me for something better. We have a 4 yr old and I don’t want her growing up without both of us, which is what I honestly feel will happen.

How do I explain my feelings to him when he knows it is both of our fantasy and holding the past over him. I trust him but I don’t think I can stand seeing him bone another girl or kiss another girl in front of me. HELP!