Cheating

My husband and I have been together ten years and for the last six months he has bought up how he would like us to have a threesome, more specifically how he wants to watch me fuck another guy.
I’ve said no but he just thinks I don’t want to go with another guy. I’m freaked out because what he doesn’t know is that I’ve already had four different lovers since we’ve been together and whilst I enjoyed the sex I don’t want him there.