Devil in the Flesh (1987) on Netflix

An Italian high school student named Andrea becomes infatuated with Giulia, an older woman he sees outside his classroom window. Giulia's fiancé, a leftist radical, has been jailed and is currently on trial for political crimes. Andrea and Giulia meet, and begin rendezvousing at her apartment, where a sexual relationship quickly unfolds. The situation is complicated when the fiancé's mother realizes this, and confronts Andrea's father. Andrea's father is a psychiatrist who treats patients with psychoanalysis. He has treated Giulia in the past. The psychiatrist finds himself a victim of Giulia's sexual advances, which he attributes to madness. As a passionate affair unfolds between Giulia and Andrea, the viewer is left wondering exactly how crazy Giulia really is, and whether Giulia had an affair with the boy's father as well. Intensity of passion between Giulia and Andrea builds towards the climax. Andrea's father confronts him about this affair, but Andrea doesn't back down.

—Wiki