There is usually a variety of onsen in the main bath - both hot and cold. During busy periods this bath would be filled with hairy (or shiny) men. Some walk around covering their lil bro while some like to show off by swinging it around ><
Being brought up in the UK - I was certainly not accustomed to walking around in my birthday suit in front of others but am now used to it - but don't think I'll ever be confident enough to start swinging lil bro around ^^
Have you tried a Japanese onsen in your b'day suit?