Heinlein's The Moon is a Harsh Mistress had a few different concepts for non-traditional marriages. The protagonist's arrangement was a "line marriage", wherei

Heinlein's The Moon is a Harsh Mistress had a few different concepts for non-traditional marriages. The protagonist's arrangement was a "line marriage", wherein the family partnership would marry in younger people at regular intervals to maintain approximate male-female parity and to keep property continuity as the older partners died off. At one point in the book, it was considered a big deal when an adult child of an older couple in the line marriage married back into it as a spouse. There was also a "group marriage" which sounded a bit like everyone in your high school graduating class marrying each other, or like a cross between a marriage and a tontine. In case of divorce, the remaining spouses would keep all the real property and the separating spouse would be paid off with cash. But divorce required unanimous approval, either of all the other spouses or all the spouses of the opposite sex, depending on how the thing was originally set up.