The plot of a straight-laced main character going to an art exhibit where they find a large nude painting of themselves that was made without their knowledge or permission and/or without them ever posing for it is a recurring trope in sitcoms. One of the earliest examples is in October Eve (1964) (Laura poses fully clothed and the artist takes license), but it also appears in The Naked Truth (1989) (Sandy poses fully clothed but finds the artist painted her head on a nude model), and What's Wrong with This Picture? (1996) (when Dana poses for what she is told will be a high-concept abstract, the artist instead paints a realist study). Each episode has a different resolution, but Julia's barnstormer speech is one of the few instances in which it is plainly stated that an artist who does this to unknowing "participants" isn't making art so much as committing a gross sexual violation.
The plot of this episode is taken from the series 'Webster', episode 'The Painting', from 1988.
A similar plotline appeared in the Welcome Back Kotter episode, "Epstein's Madonna" (1977) in which Epstein uses Julie Kotter's face on a mural of a nude woman.