Castiel's seen some lousy porn, it involved a pizza boy and a babysitter, and since this bath involved neither, he'd thought he's in safe territory. As pop culture savvy as Cas thinks he is these days, there's some serious context lacking still. Jo answer doesn't clear things up any better than him, and the angel's starting to doubt all he knows about human bathing rituals.

"And humans would prefer to be nude in public?"

Do they need the moral support? Is it for ease of having your back scrubbed (admittedly difficult to get to)? Is it an exhibitionism thing?