A Severed Head (1970)

Antonia, the pampered wife of Martin Lynch-Gibbon, an upper class wine merchant, tells her husband that she is in love with their best friend, the psychiatrist Palmer Anderson. Palmer and Antonia want to deal with the situation in a civilized way, by remaining friends with Martin. Meanwhile Martin tries to keep his mistress, Georgie Hands, a secret, but Palmer's sister, Honor Klein, who taught Georgie at Oxford, tells Palmer and Antonia about her. Furthermore, Honor introduces Georgie to Martin's womanizing brother, Alexander. This is just the beginning of the various liaisons.