I have had MMF and FMF with girlfriends, and enjoyed both.

I have been the third party in a number of husband/wife threesomes, and it can be good, or the husband can get all weird (insecurity/ego, jealousy, etc)

I have had husbands who just wanted to watch and masturbate, others who just wanted to clean up afterwards, and others who joined in and worked wifey with me.

The strangest thing that ever happened was this one wife who asked me to do her husband, because he wanted to know what it felt like to be nailed in the ass. I did.