Bob & Carol & Ted & Alice (1969)

After a weekend of emotional honesty at an Esalen-style retreat, Los Angeles sophisticates Bob and Carol Sanders (played by Robert Culp and Natalie Wood) return to their life determined to embrace free love and complete openness. Bob and Carol happily reveal their ensuing love affairs to everyone, sparking both the curiosity and repulsion of their more conservative close friends Ted and Alice Henderson (Elliott Gould and Dyan Cannon). When the two couples travel together to Las Vegas, Ted admits to an affair of his own. An outraged Alice demands that this new ethos be taken to its obvious conclusion: a mate-sharing foursome. But when they're actually ready to begin the deed, something unexplained within them prevents it.