I’m not bi, but I have had a few mfm meets where the other guy has suddenly tried to suck my cock without invitation. One time I was having sex with the lady from behind and the guy grabbed my balls completely out of the blue. I nearly hit the ceiling with surprise. Needless to say that was the end of the night.
Part of the fun is not needing to choose. I only meet couples and enjoy doing both. If I had to pick one, I'd suck as I really get off sharing a cock with a woman.
The more I'd do it though, the more I'd want to use my own cock on her.