(Classic) Never Sleep Alone (1984)

Dan and Lisa have a “beautiful open marriage” in which they are both allowed to sleep around. Things become tense, however, when he disapproves of her interest in his business partner Mark as he would rather she did not sleep with their close friends. She calls him on his hypocrisy due to his fervent interest in their neighbor Diana’s young roommate Natalie. In spite of Lisa’s jealousy, Dan appeals to Diana to arrange an assignation between him and Natalie while she is away with Lisa at a BDSM club. While Lisa whips George into shape, Dan has his way with Lisa. When Lisa finds out, she confides in Diana about wanting to get even, so Diana arranges an encounter for her with Mark. When Lisa falls for Mark and starts to see him exclusively, Dan arranges some additional machinations with Diana that push Lisa over the edge