My wife and I had two occasions where she was fucking a friend of mine.
The first time, was when I was in Air forse flight school in Del Rio, TX. One of the single guys in my class became good friends with me and my wife. It started with an alcohol induced threesome with me, him and my wife one weekend. Then it evolved into him fucking my wife whenever he wanted with my full knowledge and permission. He had a much bigger cock than me and my wife really enjoyed fucking him. He ended up moving in with us and we became a poly threesome for the last 6 months of my flight school. It turned out that we weren't the only married couple, in my class, that was doing this, lol.
The second time, was years later, when we returned to TX. This time it was Abilene, TX. I was becoming friends with a single guy in my squadron. It started off with my wife and I inviting him over for dinner. My wife made it very clear to me that she found him very attractive, lol. Like we did in flight school, we ended up in an alcohol induced threesome. That, also, evolved into him fucking my wife whenever he came over. He eventually moved in with us, as well, and that poly threesome lasted a little over 2 years. In fact, he became my wife's primary sex partner. By that time, I had realized how much I love being denied and how much I loved knowing my wife's pussy was getting fucked by his much bigger cock, everyday, instead of my tiny dick.
Abilene, however, was a very religious town. We told our neighbors that my wife and I were simply renting a room to one of my squadron mates. We did have a bedroom set up for him, but he never slept in it. The three of us shared our king bed.
Both of those times worked out very well. In fact, my wife and I still keep in touch with my classmate. He never did marry so she will travel and spend a week with him, once or twice a year.