My wife and I aren't swingers, but we have partied naked with a few couples before. We don't swap mates but we had sex in front of them, and they did it in front of us. I won't detail those, because this is a masturbation forum.
I will tell you about one get together though, that was a masturbation feast! We had met this couple, she about 38 and he about 40 out on the lake one day at a popular place where boaters meet and get together. They were fun and attractive. We spent most of the day with them, and agreed to meet for dinner and drinks that evening at our house on the lake.
They got there and we had a blast talking. They were very open minded, saying they loved nudity and exhibitionism, and they talked about places they openly fucked in public where they may have been seen. We talked about our experiences with other couples, but we let them know we weren't swingers.
To cut to the chase, we got naked in our living room, with all the blinds closed. My dick and the other guy's dick were hard as Hell. His wife was sexy! She had a gorgeous body with beautiful tits and nipples and a neatly trimmed dark haired pussy. My wife played with my dick and his wife played with her husband's dick.
The girls were basically giving us slow hand jobs. I said that I'd like to see the girls all spread open. My wife and his wife sat on the couch and spread their legs wide for us to view. What an awesome sight! I stood in front of my wife and he in front of his wife, and as we looked at them we both stroked our dicks. I was looking at my wife, then his, and so on. He was too. Finally he asked if he could stand in front of my wife and I could stand in front of his. The girls said they didn't mind.
So, I stood between his wife's outstretched legs and looked at her pussy and he did the same in front of my wife. Boy were we ever turned on! As he and I jacked the girls started rubbing their clits and fingering themselves. It was so incredibly hot!
Suddenly he leaned over and rested a free hand against the back of the couch and he started shooting cum all over my wife's stomach and tits. A moment later I was doing the same thing on his wife. The girls were saying things like 'ooooh yeah, come on Baby, cum on me.'
After he and I came, we stood there and watched the girls finish themselves off. It was amazingly hot, even probably hotter than fucking in front of and with the other couples we had been with.
We got together with this other couple several more times and we had a blast each time. He took a transfer with his job though, and they moved pretty far away. I still jack off to the memory of his wife's body, and me standing there jacking off on her, and seeing another guy jack off on my wife. It was an AWESOME experience!