Slavery was wrong because it violated "intrinsic" moral laws...that is a moral arguement. Porn is wrong because it also violates intrinsic moral laws...that too is a moral arguement.
If we applied the same reasoning to slavery as we now do with porn, slavery might not have been abolished since the arguement now is that you can't legislate morality. After all there are many people who still believe that is right to have slaves...who are we to impose our morality on them?
Now I'm not stating there is any moral or ethical way to justify slavery, just as you would state there is no real way to morally justify porn.
So what did the founding fathers have in mind when they crafted the 1st ammendment? An amoral license to print, speak, perform, any moral perversion under the sun? Or was it more a document that allowed a more lofty debate regarding morality, ethics, politics, and needed governmental/societal scrutiny?