This is an argument I've never heard before. In the US the courts have never laid down a bright red line as to what is obscene and what is not. If the porn industry were to force this copyright issue all the way through the legal system they could be in for a big surprise. They really should leave it alone and not get the public and the courts in a loud debate in an election year.
On the other hand if there is a firm legal ruling that porn is NOT obscene it means I've been downloading the wrong stuff.
It's a good argument, but it's been tried in different forms. The Communications Decency Act
in the 90's tried to make it illegal to post porn on the internet, but was ultimately struck down by SCOTA on first amendment grounds. While it's not exactly the same, the argument could be made that precedent was set that pornography can't be classified as obscene because not allowing copyrights on porn violates freedom of speech. On the other hand, I don't know what all that movie entailed, and if sodomy or other illegal acts are involved it's a completely different playing field.