Isn’t filth protected by our First Amendment?
Yes, although not completely. The Supreme Court adopted a test in the 1970's that makes it extremely difficult, but not impossible, to prosecute for obscenity. There have been a handful of successful prosecutions since, almost all of people who crossed the invisible lines observed by the industry.
Reagan appointed the Meese Commission to try to figure out how to regulate if not prohibit the stuff, but ultimately concluded it wasn't possible under SCOTUS' First Amendment case law.
Pretty much, or at least that's how the SCOTUS has repeatedly ruled.
For me, this isn't a Constitutional issue as much as it is about law enforcement priorities. You'd think that law enforcement would have more important things to worry about than some loser pleasuring himself as he watches two strangers have sex on a TV or computer screen, but evidently that isn't so. It's the same reason that I think busting people for smoking weed is a big waste of law enforcement resources.