How many households out there have Internet access? How many of those households have children? How many business with Internet access employ children? If the answer to both is less than 50% then the system should be opt IN instead of opt out.
Why should adults, businesses and the ISPs have to spend money/time abiding by this system when it has no effect on children at all in these situations.
Of course the ISPs will also have a list of people who've opted out of the block because while those other categories you refer to can be called 'adult' they are not 'porn' and they are not being blocked.
Then next, which you've failed to address, is that Davey might then decide that guns, alcohol and gambling hurt the kiddies, then probably free political discourse might also hurt the kiddies. Better get it blocked.
Finally, have you got some evidence that kiddies are getting more access to porn? Have you got evidence that they are psychologically harmed by it?