He said he wants to have a threesome with my best friend...

My "boyfriend not boyfriend" was talking the other night and brought up threesomes. I said that if we ever had one it would probably need to be with a stranger or things would be weird after. He says he wants to have a threesome with me and my best friend. I get mad and say "You're saying you want to have sex with my best friend?" And his response was that yes, he does but it's me who he has feelings for. GRR! I got mad and stormed out. This was a few days ago and we haven't seen each other but he has texted a couple times saying it was stupid for him to say. Idk what I should do. I think I've cooled off and I don't want to end our relationship, but I am so dumbfounded as to what he said.