But suppose that approach was not available to you - what methods would you implement to distinguish between pornography and eroticism, and ban one but not the other? Sufficiently clear that a scriptwriter would know exactly what they need to cut or add to a movie in order to move it from one category to the other? What if the nude "Pirates of of Penzance" was at a Pussycat Theatre and "Fuck Slaves of the Caribbean XIV" was at the Met?

Not saying that I would endorse this as a regulatory policy, but it's my understanding that the strategy used by e.g. the Chinese government is to not give any explicit guidelines. Rather, they ban things which they consider to be out of line and penalize the people who produced/distributed them, but only give a rough reason. The result is that nobody tries to pull tricks like obeying the letter of the regulations while avoiding the spirit of them. Quite the opposite, since nobody knows what exactly is safe, people end up playing it as safe as possible and avoiding anything that the censors might consider a provocation.

Of course this errs on the side of being too restrictive, which is a problem if the eroticism is actually something that you'd want to encourage. But then you always have to err in one of the directions. And if you do a good enough job at picking the people who you make public examples of, maybe most people will get the right message. You know it when you see it, so punish people when you do see something wrong, and let them off when you don't see anything wrong: eventually people will be able to learn the pattern that your judgments follow. Even if they couldn't formulate an explicit set of criteria for porn-as-defined-by-you, they'll know it when they see it.