while i vaguely agree with the idea behind this comment, this explanation is particularly poor. by that logic, it should be allowed to kill people to prevent murder. it is, in fact, allowed to kill people to prevent murder, but only in specific legally-prescribed circumstances. typically, only specific people are allowed to kill, and only to prevent an immediate murder. the same applies for child porn: it is allowed to have child porn to prevent child porn, but only under certain legally-prescribed circumstances.