A very interesting question and some very interesting answers! Apparently a very complicated issue and I'm surprised by the variety of responses.
I'm new to the site and just learning about swinging and was somewhat surprised by some of the negative responses as I'd assumed that people that swing would be some of the most sex positive and opened minded people around, but everyone has their views and that's what makes this thread so interesting.
As for me personally, I've wondered if, in a MFM situation, if there was MM contact, how would I react? If the other guy wanted to touch me or perform oral on me, could I even become aroused. So, as far as fantasizing, I guess I would call it more of an intellectual question and since I've never been in that kind of situation I have no idea if it would be good or bad. Men don't do anything for me like women do but if I think about MM contact I don't think it makes be gay or bi.
It seems like there's a double standard as someone mentioned above. Why is it OK for two straight women to be together and not two straight men? Very interesting thread as I've mentioned before and look forward to more responses.