Well isn't this an interesting thread! Since I am putting together a MFM party, I thought I would add my thoughts to the matter. To clarify the difference between an MFM and MMF gathering:
MFM (hetero) - the female is present to "service" (providing some or all of her orifices to) the men, with minimal occasions where the two males are going to touch each other sexually (any body part). The one scenario where touching might come up is during a double blow job where both men are serviced at the same time in her mouth. Some men will not allow this to happen however, and will insist on a blow job for one and a hand job for the other. The other scenario is the double back door where one guy will mounted by the lady CG and enter her vagina while the other man stands from behind and enters her greek doggy style. Having done this ... it's important to know there will be leg and foot touching by the guys LOL
MMF (hetero - bi) - the men are present to "service" the woman (the men will concede to touching of some body parts directly or indirectly). A perfect example of this is the OP photo or the double blow job scenario where the woman will actively grab both cocks at the same time, liberally cross them, blow them in the mouth at the same time, etc. I'd like to point out however, the OP photo shows bareback greek service which is super sketchy. Not realistic in today's hobbyist world.
MMF (bi) - all parties present are there to service each other (everyone's orifice is available for use).
Since many of us here on Terb have been to our fair share of orgy parties, we've all witnessed scenarios where there is close proximity of bodies grouped onto a much too small bed while everyone is interacting. In these scenarios, incidental contact happens and everybody just plays on. No matter how you look at these situations, if you are not comfortable with your sexuality and your surroundings you are probably going to find yourself uncomfortable. This is one of the reasons why I am hosting the MFM party to begin with, as it will allow people who normally don't engage in group play, to attend as a voyeur and watch. If you were at a sex party and saw a lady (like in the picture below), with a mouth begging for a cock, would you be willing / wanting to step up?