swap
Among a group of two or more couples, to swap means to trade partners for the explicit purpose of engaging in sexual relations with a different partner. See also group sex and orgy.
Example:
"Mary has always been flirting with my husband Jim. Because I have always fancied her husband Bob, I asked her if she wanted to come over and have a swap."
"Mary has always been flirting with my husband Jim. Because I have always fancied her husband Bob, I asked her if she wanted to come over and have a swap."