I've seen this topic previously in another forum. Apparently it was pretty common in YMCA's, some schools, and similar "males only" swimming areas prior to the 70's.
The hygiene part is obsolete these days, but back in the days before synthetic blends became the fabric of choice for swimwear, cotton suits would lose fibers which (given a sufficent quantity of swimmers) would clog up the pool filters. And let's not forget that every problem you can associate with unlaundered gym socks becomes about ten times worse when the garment in question is cotton swimwear being kept in a locker -- rarely being given a chance to be laundered or even fully dry out x(
It pretty much died out in the late-60s to early 70s due to several reasons which hit at the same time (more co-ed classes, rising liability rates and the ability of students to successfully bring lawsuits, increasing awareness (and reactionary fear) of gays, increasing commonness of daily showers, etc.)
On edit: make that wool, not cotton, and "hygiene" was also invoked to make sure everybody washed before going in. Found this section at
http://www.historylink.org/_output.CFM?file_ID=3100An early casualty of gender equity was males-only nude swimming in the downtown pool. Men and boys had been accustomed to swimming au naturel at the YMCA, not only in Seattle but in Ys everywhere, since the 1890s. The practice may have evolved from problems created by the long, wool swimming suits then in fashion, which apparently shed so much they gummed up the pool filters. Later, nude swimming was justified on the grounds of hygiene. A handbook in use at the Seattle Y in the 1920s required that “A good soap bath must be taken before entering the swimming pool” in the same paragraph that specified “The wearing of swimming suits or supporters will not be allowed except by permission from the director” (Information for Members).
In any case, the custom was phased out as co-ed swimming became more common, although the pool continued to be reserved for men from noon to 2 p.m. daily until 1974. At that point, Sonstelie -– then a newcomer to Seattle who wanted to swim during her lunch hour –- fired off a letter asking how an organization supported by the United Way could maintain such a discriminatory policy. “They changed immediately,” she says. “I had never seen an organization move that quickly” (Sonstelie Interview).
The new policy led to what Dick Knapp, the Downtown YMCA’s physical director in the early 1970s, called “interesting times,” since many of the men who regularly used the pool at noon had never had to wear swim suits before (Knapp, "Last Blast"). When the gender barriers fell, it took a while for all the men to get used to the new rules.