Stay As You Are (1978)

Giulio Marengo, a Roman landscape architect unhappy in his marriage, meets Francesca, a young and beautiful Florentine, and then learns she might be his daughter. He resolves to keep his hands off but can't seem to stay away, and she's eager for a lover who's a father figure. He's happy to be a kid, so he tries to find out who her father is; his wife knows something is up; his daughter, who's Francesca's age and is pregnant, encourages an affair. Should he tell Francesca his fears that it might be incest? If he tells her and she doesn't care, what next? And what of his wife, who still wants to be married? Francesca takes control.