All valid points. I was thinking from a purely technical perspective as cultural norms and varying laws are way more complex issues. That's also why I use the term nsfw rather than "adult" since it's much more easily quantifiable - anything you wouldn't want your boss to see over your shoulder.
But even on a technical level, anything that relies on the honour system is gonna be abused, so you got me there.
This is a tricky one because any method relying on the user to classify their own content as nsfw or not suffers from the same weakness.