La Débauche (1971)

Pierre is a free-lance glamour photographer who always has to hassle his penny-pinching employer just to get paid while his bored spouse Françoise stays home all day, trying to keep up a semblance of housekeeping. A former model who has apparently done well for herself, Sylvie, makes a pass at Pierre and invites him to her house in the country for the weekend. Claiming an assignment to his unsuspecting spouse, he decides to take her up on it, only to learn that she too is married to the very open-minded Michel who even brings them breakfast in bed after a night of unbridled passion . As a lark, perhaps deciding that turnabout is fair play, Michel travels to Paris to console the lonely housewife, luring her into a threesome with a casual pick-up.