Probing Research

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

Mark Roberts, Related to Bigotry: The Repression of Swingers in Early 21st Century Britain

Apparently, some of them are also footnote fetishists.