I think it may be a visual thing, for a lot of people, seeing two woman together certainly seems alot more attractive and natural than two men. I really don't know anyone who isn't gay including woman that find the idea of two men together a turn on.
Of course there's always exceptions but for many woman any fantasies they have dealing with two men seem to involve them pampering her.. not each other
Actually we got together with a group of friends recently and we got into one of those question asking modes where you go around and ask any questions you want of anyone there. The woman present were asked about their thoughts on the MMF threesome, and they seemed to think that any guy that would even participate in a MMF is gay.
Thou they were non-swinging friends (to the best of our knowledge). And I'd think it would be fair to say that they were not necessarily speaking their true feelings in that setting.
(I know this as even my wife went along with their comments for fear of getting labeled!)