You are assuming 97-98% of people 1) want to look at porn or 2) are going to bother to take the time to opt-out.
Granted, you can opt-out. So it's not like it's that big of a deal, at lest, to me. While I have looked at porn on the internet, it usually comes from starting at someplace innocent, say, a news site or something, then falling down the internet rabbit hole. (ie :Hmmm, that looks interesting. That does too, let's read that. Well now, I wonder how someone would do that, let's google it. Hmmm, something is getting past my ad filter. Whoa, what's that?" Several hours later you pull your mind out of the gutter.).
While I am not big on government censorship (ie don't like it at all), I, for one, probably wouldn't bother to opt out. By the time I remember that porn is blocked, and then start the process of opting out, my desire to look at it will probably have passed. And considering how much time I loose when I do start falling down the porn rabbit hole, I would probably start thanking the filters from saving me from hours of unproductive time on the internet.
Yeah, you can opt out. Many people will. But 97-98%? I'm willing to bet it will be under 50%. Willing to bet a good number of people are like me - while they have looked at porn on the internet, they did not have that intention when they first sat down at the keyboard, and out of those who did sit down with that intention, a good number of those wouldn't bother to turn off even a local filter, much less bother to opt-out.
It's a good plan if you want to police people's morality - while people will oppose this very vocally, most of those people won't go through the trouble of opting out, no matter how simple they make it.