Threes (1968)

A hipster swingers couple addicted to sex. He's a novelist and she's a leather fetishist who provides valuable and stimulating research for his lurid erotic adventure tales. Their open marriage allows them to travel a delirious, flesh-filled but barren human landscape where endless carnal thrills can take their toll on the soul. But that starts to take a toll on their emotions and they try to deal with it only by sinking evermore into depravity. Things can only end in the worst possible manner.