Jôfu okashi 1978

Directed by Giichi Nishihara

A man, engaged to his fiancée through a workplace romance, cheats on her with her female colleague. He becomes captivated by her sexual techniques and begins to want to break up with his fiancée. He asks his best friend to sleep with her, and his friend reluctantly agrees. They get her drunk and have sex, but this time, he falls in love with her instead...