[Proleps]

There would be a list of people who want to be able to watch porn (which could be used for black mail). It would also require a lot of infrastructure to be set up for this kind of censorship. That infrastructure could easily be used to start blocking other content.

Besides if you want to opt-out of porn you can already install filters on your computer or your router if you want to.

[foobarbazqux]

I think you reversed the meaning. Opt-in filtering means you choose to turn on the filter in the first place, opt-out filtering means you choose to turn off the filter.