Who decides (for everyone else) what porn is?
That is a good question... Something I don’t want the Government to do..
As one of the handful of people in this country who have ever had as an occupational responsibility that of determining if printed matter was pornographic ~ let me say this about that ~ if you want to mail the stuff the recipient can decide it's pornographic and ask that they be added to the list off folks who don't want it. The list costs something to create, and once a year the USPS divides the cost up among the porn mailers and charges them a proportionate amount.
Sometimes a mailer will make a mistake and send something to someone on the list ~ that's a different story ~ and there USPS will assign a classification specialist to examine the material to see if it's pornographic as understood in the law.
When that system was first set up vast amounts of porn, mostly as xerox copies, were sent to USPS for evaluation.
Copies don't count.
If you are interested in the development of the current body of law regarding porn you might try reading through all the references at >http://law.onecle.com/constitution/amendment-01/50-obscenity.html
There are some judges who like all of it; others only like some of it; and other judges who take the position that people who do not want the stuff should not have it forced on them by the government (hence the postal references throughout these decisions) ~ BUT, and it's a really big one you should all be concerned with, SOME USSC judges think you should have no right to even protest having porn tossed at you against your will.