No, there is constitutional authorization. Porn does not have full 1st Amd. protection.
While I have no trouble agreeing that porn does not have full FOURTEENTH Amendment protection, if the Court actually read the real Constitution the Founding Fathers wrote, they would notice the First Amendment reads "CONGRESS SHALL MAKE NO LAW". What basis, if any, is there for believing that this was not written as an absolute bar against federal laws regulating speech or writings of any type on any subject? To be sure, Congress didn't take very long before passing a statute which clearly violated the First Amendment (the Alien and Sedition Act) but the First Amendment serves to protect the people from such legislation.
To be sure, the Founding Fathers would have certainly seen a need for certain laws restricting the spoken and written word, but such necessary laws could have been provided at the state and local levels. Congress may have a legitimate role in aiding in the enforcement of certain local laws (nb: it's explicitly given such power to aid in the enforcement of individual states' liquor laws) but that's a bit different from imposing such laws on its own.
BTW, for those who complain that a literal reading of the Constitution would leave the U.S. defenseless without an Air Force, and would lead to airwave anarchy without an FCC, I would posit that inventions like the airplane and radio come along sufficiently seldom that passing and ratifying explicit constitutional amendments to deal with them would muddle things far less than the current practice of pretending the Constitution says whatever seems necessary at the time.