3 is a crowd?? or Is it?

Has anyone ever asked an extra man in to the bedroom? My SO and I talked about a 3 some but wit another man not woman. I honestly don't know if I would be ok with it the fantasy is amazing but I've heard before sometimes it's best it stays a fantasy. He would enjoy seeing me have that amount of pleasure. But I don't know if I could share myself like that in front of him. He said it would never hurt him once he was involved it would just be sex. He would never want me to do 3some with another girl because he knows it would destroy me to see him with another woman. But he can compartmentalise sex and emotion he said it would turn him on to see me in ecstasy. Would I feel like a cheat? Personally it would feel incredible but it may damage the relationship??

*****EDIT******

Just to answer some of the comments there's no chance i could ever allow another woman as well I just couldn't do it, I could never share him like that. That's how I feel he knows that and would never ask me to consider another woman because he knows it would kill me and he wants to see me happy and enjoying myself and not sad and uncomfortable. The fantasy is definitely becoming more curious to me.. he thinks differently to me isn't that a good thing?? Should we explore it?

And it would have to be a stranger who we both agreed on no close friends