This paper models love-making as a signaling game. In the act of love-making, man and woman send each other possibly deceptive signals about their true state of ecstasy. Each has a prior belief about the other's state of ecstasy. These prior beliefs are associated with the other's sexual response capacity...
Hugo M Mialon, The Economics of Ecstasy (PDF format) (link via Crooked Timber and Marginal Revolution).
Couples who jointly seek recreational sex with others while maintaining their emotional monogamy are most often called 'swingers'.1 Swingers' sexual habits are typically characterised by partner-swapping, female bisexuality2 and group sex.3 Full penetrative sex, though common, is not essential.4 Male bisexuality is absent.5 All swinging couples have their own rules of sexual behaviour and these come in an infinite variety from the restrictive,6 through the asymmetric7 to the relaxed.8
Swingers find each other through contact advertisements in magazines,9 newspapers10 and on websites,11 via chatrooms12 and webcam13 interfaces on the Internet, and at swingers parties14 and clubs.15
Apparently, some of them are also footnote fetishists.