Desideria (1980)

An overweight teenage girl walks in on her mother having a three-way. She later confronts her mother and gets her birthday cake shoved in her face. She then tries to commit suicide, but survives and loses a lot of weight. Now attractive to boys and men for the first time, she turns into a world-class tease, manipulating a smitten boy her age, her mother's middle-age jerk of boyfriend, an older would-be revolutionary, and so on.