You are mistaken. City ordinances absolutely comes into play here. In the US, you are subject to federal law, any additional state laws, any additional county laws, and then any additional city municipal-codes/ordinances.
Here’s the Seattle city ordinance that applies:
Seattle Municipal Code, Chapter 12A.10.130 Indecent Exposure, Paragraph A.
A person is guilty of indecent exposure if he or she intentionally makes any open and obscene exposure of his or her person or the person of another knowing that such conduct is likely to cause reasonable affront or alarm.