Mom Lover (2018)

Min-hee, a college student living with her designer mother, has two secrets.
For one thing, have an older sex partner you meet regularly. Min-hee fills her father's vacancy by meeting the man. Another secret is that she keeps getting drawn to her mom's personal trainer, Cheol-min, whom she recently introduced to her mom.
One day, when her appointment is canceled, Min-hee comes home and secretly spies on her mother and Cheol-min's sex