I worked in public swimming pool in south of France during the summer months in 2005.

The main reason the public pool where I was working started to forbid large swim shorts, was hygiene : some men wanted to swim with the same short they wear all day-long. When asked to change them, they would say “this is swim shorts” where it was indeed a regular short, but it is very had to prove .

Unless your entrance cameras filmed the men coming in wearing the same shorts they now wear in the swimming pool.

With a tigh fitting trunks, there will be less debate as they are easy to identify as obviously made for swimming, no one would wear them outside the pool.

The secondary reason was order and peace within the pool. Many bands of 10–15 young men coming from “the ghetto” never accepted to wear tight swim pants in public.

Matter of fact, they simply did not come to the pool anymore after the ban was established, leavin the pool at peace for women, children and old people.

It is not a politically correct answer, but this reason (not only hygiene) was also taken into account in the pool where I worked in order to decrease violence and maintain order.