well, I don't know, actually. I didn't know it wasn't OK to shower naked in communal showers! It could just be lack of familiarity...
I'm used to gender-segregated showering areas where nakedness is the norm.
Sometimes I do dive in, but usually I don't bother with a post-swim shower. I'll be in scruffy clothes, it's a weekend/holiday thing, & so I'll cheerfully fester in the chlorine & have a nice private shower that evening.
If I found myself in a mixed shower I might quite easily assume that the rule was 'kit off, unless you're a bit shy about nudity, which is also OK', as it is in the women-only showers I'm accustomed to, rather than 'keep your bathers on, thanks'.
I'd probably have a glance round to see if anyone else had their bum out, before whipping my own costume off, but it honestly wouldn't seem outrageous to me if they did.
So I'd be socially inept if I did strip off, on the evidence of this thread; but not automatically an exhibitionist or pervert. Possibly this bloke also simply didn't know the convention.