I doubt every guy who says they want a threesome would actually go through with it...and, going through with it also means seeking it since a threesome doesn`t just happen without some kind of proactive interest.
I think they would be intimidated when the time came, no matter how much they fantasize about it...
A lot of guys who catcall girls on the street too, would be intimidated if the girl turned around and said, "Yeah, let`s do it! I wanna suck your big fat cock, right now!"
A lot of it seems like men feel obligated to prove their masculinity to other guys.
Likewise, it seems to me a lot of women pretend to be more prudish then they are so that they can use sex as a bargaining chip and be treated like princesses; and get some doormat armcandy to show off to other girls.
However, all the pretense may also be a biological need. Women seem to need to be pressured into nonmonogamous sex and not because the bitches enjoy it any less.
In the end, I think doormats finish first. Topsy-turvy world...