The term "open marriage" came into common parlance with Nena and George O'Neill's 1972 book Open Marriage: A New Life Style for Couples. They didn't invent the concept, but they did articulate open marriage rules and offer advice about how to have a successful open marriage.
Similar to some of the guidelines for swinging, open marriages emphasize honesty and communication. Unlike the swinging lifestyle, however, open marriages allow for polyamory within the confines of a legally binding relationship. To make an open marriage successful in the 1970s - when the sexual revolution was going strong - couples had to take a leap, keep an open mind, and maybe hit up a certain type of club... together.