Lips of Lurid Blue(1975)

Original title: Labbra di lurido blu

Elli and Marco 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 surpressed 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 woman bursts into their world and Marco's deviant behavior come to light again but now he has a thirst for murder.