Also from the site
The History and Definitions of Swinging which is couples only
The history of the word "swinging" is that it was previously called wife swapping but "wife swappers" needed an easier name to describe the activity. As "wife swapping" gained popularity and media attention in the 1950's, the activity was changed to "swinging" and more recently "The Lifestyle"
So by saying your a single swinger your saying you are a single wife swapper...which doesn't make a lot of sense.
Swinging has its roots from U.S. Air Force fighter pilots during World War II. These men were wealthy enough to move their wives close to base, and the fact that their fatality rate was the highest of any branch of service led to an unusual social milieu in which non-monogamy between these pilots' wives and other pilots became acceptable. These arrangements persisted near Air Force bases throughout World War II and into the Korean War.
By the time the Korean War ended, these groups had spread from the bases to the nearby suburbs. The media picked up on them in 1957 and promptly dubbed the phenomenon ``wife-swapping.''
Supposedly there were also "key clubs" where husbands reportedly tossed their house keys into a pile in the center of the room where they were drawn at random by the wives. The owner of a selected key was the sexual partner for the evening.
Swinging has always been about couples - Every national swing convention in the U.S., and as far as I know of in the world, is couples only. Today marriage is less important so non-married "couples" have been generally accepted but some swingers prefer only to participate with married couples.
There are zillions of very good reasons why couples prefer couples. There are some single men that participate especially since clubs often have certain nights open to single men which raises lots of money since single men want so badly to participate. Those are the nights most "swingers" avoid, except for a few that do enjoy single men.
Sadly very few single women ever participate so the problem is zillions of single men wannabe's and very little demand. Some single guys do fine but they are the exception in the lifestyle. Since many women are Bi there is also a demand for single Bi women to participate, but technically whether man or women a single, is really not a "swinger" they just are allowed to participate with "swingers" (couples previously known as "wife swappers").