Constitutional amendments have limits, and rights do not exist in a vacuum. Public decency laws have been upheld by the Supreme Court even when challenged as violating the Constitution’s First Amendment guarantee of freedom of speech. As an example, it’s not legal to yell “FIRE” in a crowded theater (if there is no fire) despite free speech rights.
Additionally, when the Equal Rights Amendment is added to the Constitution, the government can still pass sex specific legislation if it has a compelling government purpose and the government action is narrowly tailored to achieve that purpose.
Public nudity prohibitions do not even have to meet this exacting standard. The Supreme Court has recognized nude dancing as expression protected under the First Amendment, YET nonetheless, upheld enforcement of public decency statutes as necessary to serve a substantial government interest.
In other words, the Court allows state governments to infringe fundamental First Amendment rights in certain circumstances. These cases provide a basis for applying a more lenient standard to test and uphold public nudity statutes challenged on equal rights grounds.