This statement is often used to ridicule an anti-pornography stance. The implication is that if there is no consensus on a definition of pornography, its effects cannot be examined. Yet there is no consensus on the definitions of many phenomena.
Rape is one example.
Legal definitions of rape vary considerably in different states. The police often have their own definitions, which may differ from legal definitions. Hence, such crimes are rarely investigated. This practice certainly has no basis in the law. If rape is defined as forced intercourse or attempts at forced intercourse, the problem of figuring out what exactly constitutes force remains.