I wonder what is the legal basis for having separate bathrooms at all. The separation seems to be a cultural one. Most of us have shared bathrooms with our fam

I wonder what is the legal basis for having separate bathrooms at all. The separation seems to be a cultural one. Most of us have shared bathrooms with our family. What practical needs does the segregation solve ? For example, if "penis sighting in the urinal" is a "problem" that the segregation solves, then self-identified gender is irrelevant.