We better decriminalize murder, because that might lead to the punishment of people who were merely defending themselves. (And, come to think of it, murder is as old as humanity itself)
Unlike obscenity though, there is at least a universal consensus on what constitutes murder, and what the justifiable defenses are (e.g. self-defense).
Secondly, murder is codified in every statute and the Model Penal Code broken down by degrees and the elements for each crime. Further, there's other related crimes (reckless injury, manslaughter, etc.) that may also fit the bill.
Our point is that while we may agree that obscene stuff is harmful, nothing can be done about it until we as a society figure out what constitutes obscenity. Then we can begin to create standards, laws, etc. around that definition.
What a crock.