With arguments on the swimsuit issue becoming more and more heated on both sides, it bears mentioning that the main reasons swimming pools require appropriate swimming attire (read: swimsuits) has nothing to do with western ideals or fashion.

The real reasons are:

Hygiene  Both because of a larger surface area and because they are multipurpose, everyday wear leaves an unwelcome supply of dirt, oil and sweat in a swimming pool.

Safety  regular clothes, especially long pants, become heavy when wet and things that are not skin-tight can restrict movement in water. Both of these pose danger to swimmers.