Personally, being comfortable with my Bisexuality, I find both man-man and woman-woman play to be incredibly hot.
I think most (straight) men find it "gross," because even admitting they find it slightly interesting would threaten their self-conception of masculinity and/or "straightness." And straightness continues to be a big deal: even today, a lot of guys who will openly admit to an interest in sucking cock - and who clearly fit the definition of Bisexuality, should they choose to accept it - still officially identify as straight. Or as "straight, but ...", which has gotten vastly more fashionable than "bisexual."
Women don't care about that as much, of course, and it does seem that a few are turned on by the thrill of provoking two men to violate their usual sexual boundaries. I have featured women like this in a number of my MMF stories. That said, the primary appeal of MMFs for most women is being the center of attention, and significant man-on-man activity within the encounter is going to end up a turnoff for most. Numerous women have said exactly this on Lush Forum threads.
I've published dozens of stories on this site over the past six years featuring man-on-man sex, and I don't get the impression that very many women at all are turned on by the idea of it happening in the real world (by which I mean outside of porn and other than confirmed gay guys).
In some ways, man-man sex might be as threatening to a lot of women as it is to a lot of men, whereas woman-woman sex threatens neither, so here we are.