The arguments against an opt-in seem to focus on giving all kids an equal opportunity to use the internet without exposure to adult content, even when the engagement of their parents may differ (i.e; the kids of parents who don't take as much interest in their development as they perhaps should can still benefit).
However, one of arguments for an opt-in model suggests that removing the necessity for parents to make this 'active choice' about what material their kids could be subject to online, just ignores the problem and, if anything, makes it worse because parents don't ever need to think about it. (Should I talk to my kids about the dangers of the internet/world? Oh, I don't care about that now, because the Government has decided for me)
Personally, I agree with you, I think it's a good idea to have an extra layer of filtering like this (not every parent is tech savvy enough to install and manage local filtering) but it has to be Opt-In. How hard is it to ask the question when you sign up with an ISP? (Sir, if you have young children in the household we suggest you choose our network level filtering)