Best friend's boyfriend asked if I'd have a threesome then backtracked?

I've known my best friend since nursery, we've always been in each others lives. Her boyfriend was a good friend of ours who she eventually got together with. I talk to him often, just normal conversation.

Today he randomly asked me how my love life was. We don't usually talk like that so I was a bit surprised. I told him it was non existent, to which he replied "wow. no sex, huh?" I responded with "that's so cringe to talk to you about lol". I thought he'd back off after that but he then continued to delve deeper into the topic, asking about my past boyfriend etc. He suddenly said "would you ever have a threesome?" I said "what a random and weird question". "Are you interested?" "In what?" " Don't get angry but I'm kinda into that and I thought that since we've known each other for so long you'd be the best person to ask." "With you and my best friend? Wtf! No!" "Relax, it was just a suggestion haha. I'm not being serious. Awkward." I told him I wasn't interested again and laughed it off. But now I'm like . I know I should tell my friend, this happened an hour ago. If I tell her, he' ll be pissed and what if it was a joke? If I don't, she'll be pissed and what if it was serious? I don't want to sabotage their great relationship.

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