I don't really get the upset about seeing people's genitals "without consent". It implies a sexualisation of all the body in all circumstances, and I think that's terribly negative.
Genitals are simply not inherently always sexual. Public bathing is normal in many places as noted. In many places family nudity is much more common. In other cultures no one wears any clothing at all. They don't go around asking for "consent" any more than I ask for consent for showing my knees when I go out in shorts. If I took my child to visit the Amazon jungle, would it be a problem that my child hasn't consented to a trip that means they might see a penis or breasts?
The main point of naturism as I understand it is to see the body as not only sexual but also simply a just a body. Old, young, thin, fat, etc. It seems to work fairly effectively at it for those who are into naturism though I think in some ways it's impractical and will never catch on widely. Though I would be all for public baths to catch on in our culture.
It makes sense to me that given the out of proportion sexualisation of nudity in our culture - the mirror of societies where women are forced to cover everything and stay away from men - naturist groups would need to be careful about attracting perverts, and as I understand it most are aware of this problem. This kind of event does seem like something that might require some thought for that reason.
As far as children being "involved" in a pervert masturbating - I don't think that's how it works. A thousand women might masturbate thinking about Brad Pitt, but he's really not involved at all.