We can't (and shouldn't) censor everything that anyone might think undesirable, but porn is one relatively easily identifiable area that has clear negative effects when "consumed" by the wrong people, such as children. And it's not just about the risk of my children being directly exposed to it, but the much higher risk that other people's children will be exposed to it, and develop unhealthy attitudes and behaviours that will ultimately lead to negative consequences for my children. It's the pornofication of society that I would like to stop, and this opt-in policy seems like a good measure to that end.If this worked 100% of the time (doubtful in this world, but let's say, hypothetically), then you could let your kids access the internet and never see any bit of porn.
Of course, they'll still have access to plenty of non-porn sites of dubious quality. Do you really want your teenager browsing pro-anorexia sites? Or how about sites that objectify one gender or another, such as pickup artist sites and sugar daddy sites? What about sites detailing how one race or culture is inferior to another?
Maybe the best thing to do is to pay attention to what your children are accessing online.
I understand not wanting your kids to be exposed to porn. It's probably not healthy for them. But there's a lot of things not healthy for children, and a blanket censorship ban on porn doesn't fix the other problematic websites your children can visit (nor does it prevent your child from seeing dubious content offline). I think having a two-way dialog with your child, and paying attention to what they are doing, is a far better solution.
(Edit: This discussion seems similar to the one we had a while back about feminism and stripping(?). Some of us prefer to focus only on the individual, while others prefer to look at the bigger picture. Just saying.)