If my wife would suggest that I would say "hooray"
Really. As long as there are no M-M contacts
I mean, I have no problems with M-M contacts between people that like it.
Generally, however, you might be right. Although I doubt that it's the case for people that have lifestyle as swingers or that often do that sort of things - I think they are both pretty liberal in sexual matters.
Why is that. I don't know. Maybe that actually is some kind of machism, but maybe it's also true that (at least I think that) there are much more bisexual women then bisexual men. Maybe I am completely wrong, of course, but from people that I know I kinda got idea that guys are mostly either straight or gay. Actually, although I am sure that they exist, I never ever met or heard about anyone being bisexual male.
And I also think that girls flirt with bisexuality much more - many of my female friends told me that they had some kind of experience with another girl, mostly it was when they were really young, and mostly it wasn't real sex, more like kissing and stuff...but, still, I think girls do flirt with bisexuality much more then guys.
Of course, you are generally right. If someone wants to try MFF threesome and his wife is okay with it but asks for MMF threesome, he should be fair and agree.