Public spaces are most definitely not "Adults Only" by default. You're not allowed to have a big billboard with porn on it, you have to show ID to get into a bar, you have to show ID to buy a pack of cigarettes, and in many states the cigarettes have to be in an opaque cabinet. You can't walk around naked. You can't be drunk in public.
Now admittedly many of these things are unevenly enforced, but society absolutely does a lot in the physical world to make it appropriate for children.
Now admittedly many of these things are unevenly enforced, but society absolutely does a lot in the physical world to make it appropriate for children.