Lips of Lurid Blue (1975)

Elli and Marco, a university professor are an unquestionably unusual wedded couple. Due to a damage as a kid, Marco harbors homosexual drives that he represses inside his married life and Elli is a barely suppressed nymphomaniac. The marriage is useful for both of them as it allows them to blend in within the confines of their regular social life in a sanctimonious provincial town. But then a strange man, George, the English antiquarian old "friend" of Marco, who now struggles to wrest him from Elli bursts into their world and Marco's deviant behavior come to light again but now he has a thirst for murder.