PG-13?

Put quite simply, those were the early days of the PG-13 rating (in fact, this film was one of the very first to receive said rating!). Obviously they were still working out what qualified for that rating and what was an automatic R (which TWIR mostly certainly would have gotten had it come out today!). Plus, things were a lot "looser" in those days and people weren't nearly as concerned about being politically correct- or as hyper-paranoid about what children were exposed to!- as they are today.

Therefore, studios were allowed to get by with a heck of a lot more in PG-13, and even PG, movies than they are today (a prime example of the latter would be another Gene Wilder film, Silver Streak. It was rated PG. Today, the studio would be lucky if they got a PG-13!).

Also, to be fair, we never actually see any FULL male nudity in this film (in the scenes in which Gene is naked, we see him either from the waist up or covering his crotch with a pillow), and the one full female nudity shot lasts about two seconds. Granted, that would almost certainly still be enough to get this film an R rating were it released today!

Yup, that's why I miss the 80s and early 90s rating system. Damn PC world we live in now.

Is there any other western country where people think 13-15 year olds need 'parental guidance' if they happen to see nude people in a movie - or else they may get harmed?

In this respect, the US falls into the same category as islamic countries with a state supported repression of nudity and sex - for the well-being of the people, of course.

Maybe American kids aged 13 actually DO get harmed when they see Kelly LeBrock in the nude. If so, it is not their biology but their upbringing that makes them so vulnerable to quite natural phenomena.