Just don't think of it as a Gender Sign, but as a Sexual Sign. The dude in the pants means you have an XY chromosome and the chick with the skirt means you have an XX chromosome.
Trolling aside, the reason why they are separate is because of what Miss Cellania said: urinals. If those were to go away, so would the sex separated bathrooms.
But if they were to get rid of those and combine the bathrooms, I, along with many other men, would probably be upset because we would be losing our express lane that the male society has gotten used to.
The only time I've seen communal urinals were at older ball stadiums and older public beaches, and both were disgusting.
Urinals are usually separate, and the process is faster is because guys don't dilly-dally.
Trolling aside, the reason why they are separate is because of what Miss Cellania said: urinals. If those were to go away, so would the sex separated bathrooms.
But if they were to get rid of those and combine the bathrooms, I, along with many other men, would probably be upset because we would be losing our express lane that the male society has gotten used to.