I'm lucky. I swing as both...
I'm maritally single, and free to do as I please, so yes I do swing as a single...
...but whilst doing so, I met a single swinging (bi) female, and one thing has led to another and we now swiing together too. (oh and she is free to swing on her own too if she wants)
I suppose that it's wrong for us to be described as a "couple" as she does have a "regular" partner, and I suppose I'm just her "bit on the side", so I prefer to think of us as a "Mixed Doubles Team". Swinging to us is more of a sport than a lifestyle, so maybe we don't approach it as seriously as some do, but when we go out to play, we do so with gusto. I enjoy seeing her having fun and she likes to see me having fun and we have great fun together too.
One thing that I do find amazing though, is how on a Friday at a club, some couples treat me (on my own) as if I were something stuck to the bottom of their shoes, whereas when they see me with my friend on a Saturday, they want to be our best friends. The two faced bastards get a piece of my mind for it too.