Adultery Diary: One More Time While I'm Still Wet (1996)

Original title: 不倫日記 濡れたままもう一度

Kumiko, who goes to a culture center with her husband's permission, has fallen prey to the instructor, Oikawa. Akiko, who also attends the same class, is single but also has a physical relationship with Oikawa. When Akiko finds out that Oikawa is also having an affair with Kumiko, she quits the class and starts a relationship with Kumiko's husband...