My problem with this line of thinking is that this is the parents' responsibility, not the state's. The government should not be censoring people's internet access (opt-out option or no) on the off chance that little Timmy might be exposed to naughty bits.
Exactly. This is a direct equivalent of banning alcohol from store shelves because they might give little Timmy naughty ideas (with the option of opting out of this restriction in which case you'd receive a super-secret brand on your forehead stating that you drink alcoholic beverages. Let's call it the scarlet letter "A").
If parents are technologically-illiterate then simply make it a formal duty of the ISP to inform users that they have the option to restrict their Internet if they want to protect their kids from stumbling upon content for mature audiences or promote the use of surveillance software that the parents themselves could use. Those programs are literally idiot-proof.
You can't ban the use of cuttlery just because a metaphorical crook might stab another person with a knife - that's just plain silly.