Alone inside one's domicile (i.e. dorm room), an individual has a reasonable expectation of privacy. A "voyeur" law applies in that case.
On a public beach, in this case a Federal Recreation Area, there is explicitly *no* expectation of privacy. Thus, open masturbation would be illegal, because its not a private place. A place cannot be both private and public. The beach is unquestionably public.
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