Wikipedia has no trouble finding people who want to contribute and there are obviously plenty of eyes on the porn. Coding up a "yeah, this is porn" button doesn't seem like an enormously unthinkable amount of effort, and I have trouble imagining it would be difficult to get users to click it.

It isn't a question of whether you agree with the law. It's a question if whether non-compliance with the law is worth the cost. The benefit of having arbitrary, untagged porn on Wikipedia is what exactly? But the cost may be having Wikipedia shut down or fined into oblivion for non-compliance. Is the world a better place with a Wikipedia with the porn tagged and optionally filtered, or with a Wikipedia-shaped hole and for-pay Britannica and Encarta sites fighting over the domain? It's quite possible that those are the real choices, and a Wikipedia as we know it today simply isn't on the menu.