Look, for everyone getting up in arms about how everyone who thinks this is inappropriate must be "prudes", maybe we can get something straight.
The naked body might be "natural", yet, for some reason, we still have laws about not being naked in public. We don't let people stroll around starkers in the streets.
There's absolutely nothing wrong with this family choosing to parade around naked either in private, in their own home/lands or in specific nudist areas.
It is absolutely and totally unacceptable to suddenly announce - after a holiday has been booked - ESPECIALLY when there are teenage children around - "oh by the way, we like to stroll around naked - that's okay, right?".
That isn't a social norm - it's not wrong, but it is wrong to expect others to turn around and say "oh yea, sure".
Japan has all their lovely onsen, which require total nudity, with absolutely zero privacy. Despite that, nudity outside of the onsen is strictly prohibited.
You don't have to associate the body as something shameful, whilst saying that it isn't appropriate to be naked in many circumstances.