What exactly makes consumption of porn—or, alternatively, compensating women for the production of porn—a “degenerate activity”?

Id's say "degenerate" is a catch-all for any activity which produces a supernormal stimulus, far in excess of what our Neolithic biology would be prepared for. Excess food, drugs, sexual novelty, and gambling all fall into this category.

You could certainly describe excess consumption of food, drugs, and/or sexual novelty as degenerate behavior. But GP appears to simply be referring to either consumption of or payment for pornography as a degenerate activity.

The argument is any modern pronography represents an excess in sexual novelty compared to the baseline.

That is more or less a completely indefensible position given the wide gamut that exists today.

If you take the baseline to be a Neolithic life experience, then I don't think it's indefensible at all.