To be a bit more explicit than badrabbit about why the symmetry isn't there:
The problem a filter like this is meant to solve is: You go on the web reading random stuff, and occasionally you run into something pornographic when that isn't what you're after, and it's unwanted (because you're at work, because you find pornography disturbing, or whatever).
So the problem its inverse would solve would be: You go on the web reading random stuff, and occasionally you run into something non-pornographic, and that spoils your fun. Or maybe: You go on the web looking for pornography, and occasionally you run into something non-pornographic, and that spoils your fun.
So far as I am aware, those are not problems anyone has.
Variations on the theme: You might be looking for some specific variety of pornography and keep finding pages with different kinds. I don't think this sort of tool would help with that; all it can do is filter out unwanted bits of pages you've already found. Or you might be successfully finding pornography but not want to see any other clutter -- e.g., advertisements on your screen distracting you from it. But I'm pretty sure the only people advertising on pornographic websites are highly NSFW themselves, so I wouldn't expect a reverse-NSFW-filter to help with that either; what you want for that is an ad blocker.
> So far as I am aware, those are not problems anyone has.
Rare moment I can chime in to the opposite, this is an issue and there is an anti-sfw filter built in to some sites.
The use case is for imageboards primarily where trivial arguments spring up easily, the "fap time" sfw filters remove text only posts and posts without nsfw imagery.
The problem a filter like this is meant to solve is: You go on the web reading random stuff, and occasionally you run into something pornographic when that isn't what you're after, and it's unwanted (because you're at work, because you find pornography disturbing, or whatever).
So the problem its inverse would solve would be: You go on the web reading random stuff, and occasionally you run into something non-pornographic, and that spoils your fun. Or maybe: You go on the web looking for pornography, and occasionally you run into something non-pornographic, and that spoils your fun.
So far as I am aware, those are not problems anyone has.
Variations on the theme: You might be looking for some specific variety of pornography and keep finding pages with different kinds. I don't think this sort of tool would help with that; all it can do is filter out unwanted bits of pages you've already found. Or you might be successfully finding pornography but not want to see any other clutter -- e.g., advertisements on your screen distracting you from it. But I'm pretty sure the only people advertising on pornographic websites are highly NSFW themselves, so I wouldn't expect a reverse-NSFW-filter to help with that either; what you want for that is an ad blocker.