Though I may personally disapprove of online pornography, the fact is that it exists and cannot easily be prevented outright.
The laws against child porn, in particular, have resulted in, at any one time, literally thousands of people being sent to prison for viewing images of those under 16 or seeming to be of such an age. Most of those sent to prison are entirely harmless and, moreover (because such crimes are notionally "sex crimes" are not sent to open prisons. A waste of public money for nothing.
My proposal would in fact assist the police in catching child abusers, because the decriminalization would flush them from the covert.