It's true. People don't realize it, but chances are you have a friend who is a swinger. There's not a LOT of swingers, but somewhere around 1 in 20 or 30 are. Do the math. I you have enough friends, one of them is likely a swinger.
This came to pass with a dear friend of ours. He'd been with his girlfriend for some years, and they were beginning to experiment with sexual escapades beyond themselves. We're quite close with this friend, so he shared some of this with us. This was coming up on Christmas. A week or so later, we wrapped up a book we had read on swinging that was quite informative and gave it to him. When he opened it, he was astonished. We said "This was our book. We're passing it on to you now, as we think it will be quite useful." His jaw fell on the floor and he said "YOU?!?!? NO WAY!!!" It was hysterical! My wife has never played with him, and he and the other lady broke up about a year after this (unrelated to swinging).