Porn is defined as #2354 in the Catechism. It is made all the more offensive and heinous when it includes minors, for example.
For us Catholics, yes. As a Catholic, I agree completely with that (and that's what governs my personal belief on the topic).
However, Catholicism is the religion only 80 million or so of the 300 million who live in this country, and it wouldn't be an illogical guess that a good number are lapsed or non-practicing. While that makes us by far the largest single religious group in the U.S. (a full quarter of the population), that doesn't give us free rein to implement that across the board.
This is the onus of why I believe we need to have a secular definition of porn if we are going to combat it in this country. Do I believe that porn needs to go by the wayside? Absolutely.
But I also believe that if we are in disagreement as to what needs to go, then it's wasted effort on the part of all of us. Whereas, if we define what smut is, it makes it a lot easier to regulate and/or eliminate.