I'm asking this in good faith.
Given that:
1. The Internet is not an optional subscription service today the way it was in 1995. Every kid and adult has 1,000 opportunities to get online including on the multiple devices every one of their peers owns, which a single set of parents has no control over. So "Just keep them off the Internet/control their devices" seems like a silly "Just" instruction.
2. The Internet is nearly infinite. The author of this editorial says "then install a content blocker on your kids’ devices and add my site to it". This is a silly argument since the whole point is that no one has ever heard of him/her and it's obviously impossible for a filter (let's just assume filters can't be bypassed) can "just" enumerate every inappropriate site even if it employed a full-time staff who did nothing but add new sites to the list all day long.
So given all of that, how do we justify how the Internet must operate on different rules than the offline world does? One can't open a "Free adult library" downtown and allow any child to wander in and check out books showing super explicit porn. I'd have to check IDs and do my best to keep kids out. It also seems like it would be gross to do so. If you agree with that, why should the Internet operate on different rules?
I'd also like to separate the logistics from the morality here. If you believe it's hard to do it without satisfying privacy concerns, totally true! But then the focus should be on finding a good privacy-respecting solution, not just arguing for the status quo.