This is common behavior in many places. Some have both gender nude bathing, some have male only & and female only bathing

I've been to hotels in Japan where the baths are same gender nude bathing, soaking, sauna. Same thing in China and Korea.

In Germany the famous Baden Baden spas have same gender days and mixed gender days. All totally nude.

And, for those of us who have been around a while there were decades when swimming at the YMCA or taking swimming lessons at a college meant you were swimming naked. Right here in the U.S,

See e.g. http://www.vocativ.com/culture/fun/f...swimmers-nude/

Note: you will see a couple photos of naked butts in the link above.