One of the precepts of the rule of law is that the laws apply to everyone equally. And the Equal Protection Clause of the U.S Constitution prohibits states from denying "any person within its jurisdiction the equal protection of the laws." So when you have a law that only applies to women (like, for instance, Fort Collins's statute saying, "No female who is ten (10) years of age or older shall knowingly appear in any public place with her breast exposed below the top of the areola and nipple"), that might be a red flag that a law doesn't apply to everyone.
The good people at Free the Nipple sued Fort Collins, claiming the law discriminated against women. And a federal judge found that the group was likely to win their case and granted them an injunction to keep the city from enforcing the law. You can see that order below.