It's very cultural thing I'd say. I was raised in a family of nudists. We didn't lock ourselves while using shower and we as a family spent every summer in nudist camp or at least went for a swim on nudist beach. FKK = Frei Koerper Kultur if I'm not mistaken, interestingly began in DDR (former East Germany). But I do remember some of my classmates in 70's were taking shower in swim suites. Within their homes...
What I want to say is that when you've been in a place with so many naked bodies it isn't anything special or of interest anymore. You just do what you have to do (take a shower) and out you go. You don't have to look at others because you already know they would usually have four limbs, skin over their flesh and that's about it