Q. What's this all about?
A. I'm not entirely sure of the specifics or the protocol, as I've only ever seen one and heard about what happens inside second hand, but apparently you pay a fee to rent a towel (ew) and then you're free to wander around naked at your leisure and stare at other naked gay men at their leisure.
Q. This sounds like a recipe for AIDS. Is it?
A. Again, I'm only speaking out of conjecture, but it's as reliable a recipe for AIDS as Hamburger Helper is for sodium-filled starch.