We do mfm, always with this one guy we know. Weird, awkward any scary first times, much better now.
We had an unplanned hook-up with a guy out of town once. Met him in hotel bar, was HOT and exciting in the bar leading up to joining him in his room but it kinda fizzled out. We were nervous, he was kinda bossy so it didn't go well. He did do her while I watched but it wasn't fun for her or me.
We hooked up with another couple once on a cruise, was pretty good I will say, we all got along and we did swap wives on the same bed. I did enjoy sex with his wife but I was distracted trying to watch my wife.
We haven't really discussed or agreed on anything but we like our regular, it's comfy and relaxing for us.