A Very Perfect Couple(1974)

Angela and Robert have been married for ten years and make a perfect couple. They are madly in love with each other, give each other plenty of space and support each other in everyday life. This idyll is put to the test when Robert's girlfriend Claudia moves in with the couple and Angela falls in love with another man. According to a mutual agreement between Angela and Robert, anything goes if one of them breaks their promise never to leave the other.