What I have found with mfm is if you stick with the major rule everything should run smoothly. Meaning, always stick to North and South. If one guy is North the other should be South and vise versa. However, DP can be tricky. Just need to take your time with that. Not sure how many women are open to DP anyway. (No pun intended).

no crossed swords!  Seriously, DP seems hot because it is such a wanton slutty thing for the girl to do, just like 2 cocks in one pussy or ass as some porn shows... but as a straight guy to contemplate the amount of contact it requires with the other guy (much less actual dick to dick in dual pen one hole, yikes!)  I would imagine boner deflation right away.  Hotter to watch than participate in.   OR, keep to the North South rule, avoid crossed swords, slapped balls, rubbed thighs etc and just touch the girl. That could be hot.

I think this goes to the casual bisexuality of women being much more common... its easy to think of 2 girls sharing sucking a dick and having lip contact, even if not enthusiastically bi, and very hard to imagine straight guys having casual lip contact while sharing a pussy.    

I think its because Men are generally the seekers (we're the ones shelling out $$$ here) and Women the sought resource.   Two women are peers in being wanted, two men are competitors in wanting.  Thus the great No-Homo wall?

of having the smaller dick of the two

A lot of CL ads that post looking MW4M note, not gay/bi but some incidental contact possible.  If you want to try DP, some incidental contact def likely.  Some guys, on first time, might be wary of performance anxiety. Others afraid it might expose or unleash an “inner gay” inside them.

No real interest

I believe most men don't have any real interest in MFM.  It's fun to watch, but really most men want full access to a women's body.  I don't want to kiss a woman and then have her kiss another guy within a few seconds.  I prefer to have her to myself (or with another woman) versus with another man.  It's not all that sexy in reality.

IMO, if a guy is that concerned about being 'gay' just because there might be contact, he probably shouldn't be doing MFM.

I'm straight and I've had a few MFMs before and I've always had a good time. I don't mind incidental contact--I mean, come on, it's sex, and if you're doing an MFM you shouldn't be THAT allergic to male contact. I've never done a DP before, but if there's incidental contact, again, I don't think I'd be too worried.