My DH was told he couldn't go in water in South of France wearing his shorts-style trunks. Had to buy skintight ones, he hates them. The explanation we were given was that it's the law in ALL communal pools in France and is because teenage boys lie around in damp shorts all day and it's unhygienic.
Weirdly, it was hard to track down a pair of the right sort in French shops. Would def buy before you go.