The problem is that once you start blocking content of which you disapprove the door has been opened to block all manner of content -- especially political dissension.
If one group thinks a particular subject is reprehensible there will be others who consider it valid and legitimate -- and that applies to porn.
Give religious fundamentalists the right to block content that offends them and they'll ban all atheistic content and then will extend the ban to all scientific content that doesn't fit in with their creationist mythology.
But porn is a special case because there's no effective way to keep young kids safe from it. And yet, porn is so popular (even among people who decry it) that it generates more income annually than Microsoft, Apple, Google, Facebook and Twitter, combined.