Sexy (1987)

Dr. Melissa Dowler, a porn star turned hands-on sex therapist, instructs a couple and later gives client Tom his first sexual experience. After her neglected husband David convinces her to let him watch through a two-way-mirror, she instructs a threesome and helps another client Larry contain himself via mutual masturbation. Having seen enough, David barges in as a fake doctor to take advantage of the final client Sandy who never had an orgasm.