I am totally straight, but actually I only had mmf 3p.
Here is my thinking, It is much more hard for me to enjoy 2 girls, than for 1 girl to enjoy 2 guys. From a lot of nice experiences, I find it very exciting when you get the girl sucking you to see her also get banged hard, or opposite when you bang her to see her suck a guy, like having your live porn while fucking. Surely it's for people enjoying also parties/orgies with like 5 or more people, 3p is no issue.
Actually it also get an advantage, while you switch positions, the girl keep enjoying, you easily get a 4h sex session without interruption, while alone I would never bang a girl for that long
In any case, you're never close enough to the other guy to see his cock in detail, and why would you stare at the guy when you got a very horny girl
I never gone to double penetration or close proximity with the other guy, which is I guess what would freak me out, and many people readig your answers to the thread. I avoid any proximity or contact with the other guy.