While you disagree with the other side at least you see that there is a disagreement and are ok with it being that way. Others would rather force their side, regardless of which it is on the other. And I'd disagree; here in the US you'll find that both sides can equally be prudes which is where this conversation was heading. I may disagree how you'd raise your kids but I sure as hell won't be telling you how to raise them. However, if there is some tool I could use to keep mine away from things I deem as unnecessary for them to get involved in that is my prerogative and I'll use it. Just because another tool becomes available doesn't mean that *everyone* has to use it. The bigger issue I see with this is if we start to segment the internet due to one sector this will give governments, private companies, etc to effectively do what China does and remove things that those in charge don't deem "fit" for anyone or will use it as a propaganda machine.
I think if it was done on the ISP end it would encourage competition since not all ISPs would have to have adhere. And ISPs DO have the ability to do this. Whether it is legal or not is another issue, as you'll have those that don't want it. Although if it were opt-in it wouldn't be a huge deal; they'd have to use static IPs most likely. Or just put certain subnets on it and make sure that those that opt-in only pull IPs from that pool.
If it's not hurting your liberties I don't see the issue in the end. Keep in mind your liberties don't extend into raising my child for me which is why I disagreed with her. However, I don't see giving the option she suggests is infringing on anyone viewing the internet since it is opt-in not opt-out. It does put a burden on the ISP, but I'd suggest they just opt some policy of "if your kids sees porn on the net we're not responsible" deal. Since ultimately it is up to the parent to supervise their kid.
A while back I had a mother ask me how to supervise her teen on the net and I basically told her to stick the computer in a room where everyone is normally and to just monitor usage when the kid is on the net. Beyond that I basically left it in her hands to do her job as a mother. Oh and I suggested removing power from the machine while she wasn't home; unless the kid went to buy another power cord or something.