While she was using the women's locker room at a fitness center, she saw another member who was unclothed and thought this other member was a man using the wrong locker room. As it turned out, however, the other member was a transgender woman and had been mistaken for being male, in part perhaps because of the absence of traditional female attire and the presence of male genitalia.
People using single-sex public restrooms, of course, do not ordinarily expose their genitalia to other restroom users, so I don't think the same confusion is as likely to arise for restrooms. The locker room example likely presents a much less common situation, but it probably cannot be written off as an absurd hypothetical either.
This blog reflects the views solely of its author. It is not intended, and should not be regarded, as legal advice on how to analyze any particular set of facts.