My theory was always that they think it deters perverts. They figure a pervert who'd get 'excited' by the nudity would be put off by the fact that his 'excitement' might be clearly visible. Or to put it in a broader context - they feel nudity as a communally shared element is asexual and 'pure' whereas the presence of a clothed person introduces an element of shame and sexual friction.

There's of course a long history of nudism in Germany and ties in with a cultural obsession with the 'natural' (see organic food, anti-nuclear energy movement, popularity of hiking, esoteric movements etc.) that to that extent doesn't exist in the rest of Europe or the 'West'.