I used to know some people who were into this. It was by no means the lovely, consensual fluttery idyll being delineated here.
I didn't go to any clubs with them. They tried to persuade me many times to have sex with them both. I didn't want to because the man was a creepy git. They handed me pieces of paper with websites about sex, which I didn't investigate.
Eventually the woman made a move on me when he was out for the evening, and I went along with it to see how it felt. It was kind of a non event, I mean, it was pleasant being tactile (she was an old friend) but I didn't get off on it, though she seemed to.
Then he got home and I ended up sleeping in their bed for the night as he was too pissed to drive me home - and he was sleeping with her, and tried to get it on with me as well but I wasn't really up for it though I tried to be polite and let him mess about for a few minutes (nothing 'happened' with him, technically thank God).
He wasn't pleased about this but made out he was fine with it...he was insanely jealous of us doing it without him, and I felt frightened being driven home by him after. Their relationship failed shortly after when he started throwing her possessions off the roof.
He was a violent alcoholic. He also used to walk around wearing the shortest shorts you have ever seen, in public, and had custody somehow of his children claiming that their mother had tried to murder someone.
That's my experience of the swinging scene. No thanks.