Come on, that's a very lazy answer. I'm in CA and have never heard that visiting a nude beach can get you on that registry.

It's a weird grey zone of laws where the beaches are not officially nude beaches, but they are advertised this way. Many are run by the federal park police. Most anti nudity laws are state laws and as a result, there is kind of a loophole with enforcing it.

Of course, the act of being nude in public can make many believe they have been assaulted when it's just nudity.