More and more bathrooms are becoming gender neutral. This is a good idea as our understanding of gender is becoming less binary. This tends to mean more privacy such as single unit facilities which can also add to the cost of buildings.
In the past, like the 1950s, facilities were divided by gender, but they were still public spaces to save on construction costs. Pragmatism.
I've got to thinking that for restrooms and especially spas, saunas and showers, there could be two types of facilities. Instead of men's and women's, it would be "private" and "public." Often spa, hot spring and sauna type places are social environments so some would be public for that reason. Others would be private. They could all be gender neutral, however which solves some problems.
Some public spas would be clothing optional while others could continue to require bathing suites.
This could be the future for these types of facilities.