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..