I think you're seeing things that don't exist, at least not frequently, perhaps because you've been exposed to people promoting that in as the opposition/justification to their own extreme behavior.
I've given you reasoned objections to sending your kids to nudist camp, not hysteria. Yet you insist on claiming that the objections posted here are "hysteria" from "bad ideas about sex, nudity, and the body". Where's your evidence? If you have none, I suggest you form a new conclusion based on what evidence exists.
And FWIW, I think that if there is 1% that conform to your image of the sexually repressed conservative, they probably exist at 10 times that rate here.
My objection to your "reasons" is that you present your "reasons" as universal norms with absolutely no justification, when in fact they are simply artifacts of our culture. I've said it earlier that it's a simple objective fact that the justifications you present are not universal among all human populations and hence the problems you cite stem not from the activity (naked teens being around each other) but the cultural context in which the activity is occurring (a society which regards nakedness as wrong or shameful) - which I think illustrates a problem with our culture.
Now you may have a point that I hang around FR too much and tend to think that the views here are normative of mainstream American culture - but at the same time I can easily compare modern mainstream culture to eras in the recent past and understand that it's more restrictive now - perhaps not as much as say the 50s but certainly more than 70s - you think a movie like "Last Tango in Paris" would be made in today's PG-13 Hollywood?