first used by Nena and George O'Neill in their 1972
book Open Marriage: A New Life Style for Couples
. Unfortunately, the way the phrase is used today (and in dannye
's former writeup in this node), for marriage where the partners can have sex with other people, is not the way that the authors meant it; only one chapter in a 270-page book even mentions sex
with people outside the marriage
The idea of the book is to build marriages that last, by throwing out the older ideas of marriage, which the O'Neills call "closed marriage" where the partners have to be everything to one another: "The clauses of the closed marriage contract dictate that Gwendolyn and Richard must always appear together as a couple, must share the same friends and forsake those the other can't tolerate, must always share vacations and most hobbies, must always be available for each other's whim and loneliness, must both put their moneys into a conjugal financial pot and must never feel attraction to anyone of the opposite sex." (It also spends a lot of time on strict gender roles and how they handicap people from having a good relationship.) "In a closed relationship, the couple does not exist in a one-plus-one relationship. Their ideal is to become fused into a single entity -- a couple."
The open marriage idea allows people to be individuals first, without having to be "the object of total validation for the other's inadequacies and frustrations." It allows people to have interests and growth as people outside the couple, but requires a lot of communication, trust, and rewriting what people expect of romance after years of unrealistic portrayals in all sorts of media.
It's quite suprising how many of the ideas from this book have been adapted by other relationship self-help manuals and gurus (or reached independently by them). The book is a bit dated, being almost thirty years old and aimed at a world still reeling from the beginnings of the women's liberation movement. But some things in people's relationships haven't changed, and perhaps they should. It's a shame that one chapter about sleeping with other people (offered as a possibility, not a requirement) is all that most people seem to have read and all that this phrase means to the world now.