There have been several articles on this topic. Most recently was in the L.A. Times entitled "A drawing class -- or adults-only fare?". I've been told Long Beach, parts Santa Monica, and L.A. are implementing these ordinaces.
"City offices are saying that life drawing classes which charge a fee for service are considered to be adult entertainment. Adult entertainment venues are strictly forbidden without a city permit. Even with proper permits most life drawing classes are closed down because many are in close proximity to residential property which is forbidden by law for business of adult entertainment."