Because it's not sexualized nudity but just...nudity. Hey, wanna see what germany's most popular teen magazine looked like? There was a section called "Dr. Sommer Bodycheck" where two teens in full frontal nudity stand there, with the magazine explaining different details of their body. Check this out, from their still functioning website:
A Vulva Gallery! (Click on "Gallerie starten" if you're interested, nsfw obviously)
I've seen naked children and babies on TV, be it during a TV show or some reality programming as a kid here in germany. Criminilizing nudity as if it is some intrinsically "adult" thing children or teenagers must not be exposed to is incredibly silly and even counterproductive when talking about sex education. This movie wants to realistically portray the struggles of a Transgender teenager, and their coming to terms with and accepting their body can be a central theme of that story. To then shy away from showing that reality for no reason whatsoever but a criminilization of nudity, no matter if its educational or otherwise non-sexual or not, is actually pretty offensive.