ˌopen ˈmarriage noun [countable] a marriage in which both partners accept that they will have sex with other people
[TahlilGaran] Dictionary of Contemporary English ▲
open marriage n. phr. An arrangement by mutual agreement between husband and wife whereby they are both allowed to have extramarital affairs.
Chances are the open marriage arrangement they had didn't work out too well so they are getting a divorce. [TahlilGaran] English Idioms Dictionary ▲