wife-swapping 
noun [mass noun] informal
The practice within a group of married couples of exchanging sexual partners on a casual basis: what do you think we go in for?—wife-swapping? [as modifier]: a wife-swapping club...
- While basic lovemaking sessions remain, the survey found marginal behaviour such as wife-swapping, spanking and group sex was on the increase.
- These folks realized that they enjoyed having sex with multiple people, enjoyed wife-swapping, and see nothing wrong with it.
- Meanwhile, insiders say that the couple's 18-month relationship may buckle in the wake of sordid wife-swapping reports that emerged last month..
- multiple birth
- multiple bond
- multiple bonding
- multiple chemical sensitivity
- multiple chemical sensitivity syndrome
- multiple-choice
- multiple-color
- multiple-colour
- multiple-disc
- multiple-disk
- multiple-drug
- multiple echo
- multiple exposure
- multiple factor
- multiple fission
- multiple fruit
- multiple gene
- multiple image
- multiple injuries
- multiple integral
- multiple-launch
- multiple listing
- multiple-mirror
- multiple myeloma
- multiple neuritis