how does your average parent, with a basic grasp of the internet, protect kids like you have done with yours?
Talk to them, keep an eye on what they're doing, take an interest.
And what would be the point if they can access stuff at their friend's house?
Porn site are a heavy business. They're absolutely laden with malware, so machines which regularly access porn are likely to acquire all sorts of coughs and sneezles. That alone makes a "not under my roof" policy sensible.
I'm not sure (and perhaps it's my middle-class naiveté) that collaborative porn watching is a thing, as porn is usually a fairly solitary occupation, and I'm willing to gamble that is X% of children have access to porn at home, only (X/Y)%, for quite a large Y, are willing to show it to their friends. They would, for example, then be reliant on their friends not telling their friend's parents. "Oh, they just look at it at their friends' houses" seems like quite a considerable cop-out, and I'd want to see some evidence that this happening at anything like the rate of people viewing porn alone in their own houses.
Why can't you let parents adopt the simple measures that, whilst not perfect, will protect some children.
The onus is on people proposing inadequate measures to show that the benefit outweighs the unintended consequences. I don't think that case is remotely made that the simple measures are of any significant value, and I think the false sense of security is a serious problem.