Men whose partner (F) wanted a threesome with another woman. Why did you say no?

I know it’s silly to assume that it’s every guys fantasy to have two women at once, but I feel growing up it was something a lot of us boys fantasized about(?) I’ve been told by a couple female friends and ex’s that their (previous) partners have turned down the opportunity or said no to the idea of having a MFF threesome. So I’m just curious as to why? Not trying to shame, just interested in hearing different perspectives