engaged and he’s wanting a threesome...

so my fiancé over time has wanted a threesome because he’s never had one, but I have. I tried telling him it’s truly not all it’s made up to be, and personally I could care less to ever have one again. but it’s been brought up multiple times since then...i told him awhile back if he really wanted one then we could because I feel that if we don’t before we get married he will still be curious and resent me for us not doing it.

so while taking about how there would be no rules it would be like a free night to let go and get nasty. no point in having boundaries or rules in the bedroom if that’s going on bc I feel like it would just cause problems of jealousy. anyways, after talking about everything he was like idk never mind because what if I don’t look at you the same after, or you wanna leave me...so we dropped it.

then last night he brought it up to me again wanting to do it. idk what to do and I need advice!!!