You're supposed to wash up your body before entering an onsen. Even though you're technically there to bathe, you're not supposed to enter the onsen dirty for spiritual reasons (or so they say, but who wants to get into a bathtub filled with dirty water anyway).
However, that got me thinking. Do people wash up their foreskin and buttcrack in public before entering an onsen? Anime never shows it, and if the purpose of washing before entering is of any importance at all, I think not taking care of those two spots counts as a major design flaw of this custom. Onsens are probably filled with sharticles.