Although I agree with some of what Bricker says here and, as always, his legal analysis is impressive, I think the OP makes some good points.
I*m quite convinced that the proponents of this law did so in bad faith. As the OP suggests, it was partly (or even primarily) designed to impede the production and distribution of porn featuring adults through onerous recordkeeping requirements.
The history of this legislation shows that prior, more honestly drafted statutes attacking all porn distribution were struck down on First Amendment grounds. As impeding internet porn through the front door failed, the drafters of this law chose a back door, dishonest method.
The method? As court precedents allow bans on child porn (for reasons that make sense to me), a law supposedly meant to better accomplish such a ban would survive a First Amendment challenge.
And it worked. Many secondary distributors of adult porn shut down rather that comply. Even sites with incidental nude images took them down rather than worry about renning afoul of the law. This makes the drafters of this law very happy.
I believe that there were and are less restrictive methods available to combat child porn. The Traci Lords situation is so well-known because its an aberation, not because it illustrates a common problem with mainstream porn. Even if this was a widespread problem, whats wrong with allowing the small scale vendor to rely on the records maintained by the producer of the image?
Make the producer maintain the records and certify to downstream vendors that all actors are over 18. What*s wrong with that? Put some teeth in the law to punish producers who lie.
And for the small-scale self-produced images, make the law create a presumption of guilt (of child pron) if the records arent kept that can be rebutted by proof offered by the defendant that the model is indeed 18. Dont make failure to keep records a crime in and of itself.