Threesome?

My boyfriend and I were talking last night and he says that he would never want to do a threesome.. but if I wanted to he would for me. Like it’d be him, me and another woman… im not down for threesomes and don’t see myself ever being down for them even if it were two dudes and one girl.. and he knows that? Like is that secretly something he wants to do?? I was just kinda like uhhhh I’ll never ask you for one that’s not something I’m comfortable with and he then rethought it I guess and was like no I only need you anyway I wouldn’t do it even if you wanted to.. like umm what you literally just had some sort of fantasy?? Idk what do y’all think? Do people fantasize about threesomes in monogamist relationships?