Maruhi kyôsei-zai: Bed ga nureru (1980)

A woman works as a nude model for vinyl books. She breaks up with her cameraman, and starts living with a cute male university student. One day, she goes to the supermarket to buy a change of lingerie for a shoot, but realizes she doesn't have the money, so she shoplifts. There, she meets a middle-aged man that also shoplifts. She goes out drinking with the middle-aged man because of their affinity, and they end up in bed. The two start dating. The woman decides to invite the middle-aged man to her room because she thinks the two-way relationship between a college student and a middle-aged man is fun. When the two men run into one another, they are surprised. It turns out they are father and son! From there, the strange cohabitation of the three begin... And that's just the tip of the iceberg.