It's just a culture. I have seen whole Japanese family strip naked and go for onsen together. No big deal. In some parts of Bali, the whole village go to the communal bath, strip naked and wash together (now getting rare, though, no thanks to tourism).

It's best to mind our own business and keep our "dirty" thoughts to ourselves and not impose our "standards" onto other people. 
(Child abuse is a different matter, however, but let's not go into that).