@Linkat,
I know exactly one couple who swings (so not much of a dataset). However, my understanding is that swingers have social groups to meet other couples, and that much of this is now done online. I can't imagine they would pick up other couples in public unless there was some reason to think you were open to such a thing.
There are resorts for this sort of thing, where every couple there is open to sex with others (or they wouldn't be there). It sounded awfully awkward to me.