The After School ONANIE (1990)

Original title: ザ・放課後ONANIE

A high school couple. The girl's parents are currently working overseas, so she lives alone in her parents' house. She and her boyfriend are in love and having sex all the time. Her boyfriend is working hard at his part-time job to buy her a present for her birthday, but when she says she wants to go on a trip, he needs to save up more money, so he ends up going along with a fraudulent investment plan, and ends up in a lot of debt...