Nice Guy Private Eye.

A married woman, Kazuyo, enlists the services of the private detective, Rokusuke, to see if her husband Norio is cheating. Kazuyo says whenever her husband cheats on her, he then comes home and has sex with Kazuyo, to hide it. However, since her husband broke up with his mistress, her own sex life has taken a big hit. In her frustration, she seduces detective Rokusuke. Soon after that, Norio starts cheating again - this time with another woman. The woman bears a striking resemblance to detective Rokusuke's wife. Now, he's faced with a difficult choice.