Well child porn serves as a good example. Tagging it would be effective in monitoring its diffusion, but it's not a neutral thing or a victimless crime like drug consumption; any kind of photographic child porn includes a child whose identity and sexual abuse are the subject matter of the material, and every time it is collected or viewed the rights of that person are violated anew, even if they have since reached adulthood and are thus of no further direct interest to child abusers.

I'm not sure exactly what technical measures should be employed in relation to such cases - perhaps machine-curated honeypots whose attempt to access is itself a crime.

CP raises extremely tricky moral questions. I mentioned photographic examples above because they include real people. But there's a tricky question of what happens in relation to creative works not based on reality such as drawings. It's easy to conceive of art that that would automatically be deemed illegal or at least prurient if it were photographic, but since art can be created from imagination without any real person needing to serve as a subject it's very difficult to figure out an appropriate response.

We could say that the simple lack of actual human victimization should OK the production of such content, but form a practical point of view it's possible to create hyperrealistic art works that are almost indistinguishable from a photograph, and computers are making it easier and easier to produce renderings that approach photographic quality, a trend which will only continue because of the huge economic incentives to instantiate virtual puppets rather than pay people to work. I'm sure this has been employed to produce crude CP already (based on its use in non-contraband porn). It's difficult to make an argument for expressive freedom when the only expression involved in its creation is that of the desire for activity that's illegal, but on the other hand nobody would think of arresting someone for a painting that depicted one person stabbing another notwithstanding the possibility of the inspiring idea being illegal.

This issue matters since it's a demonstration of art's power to subvert norms by realizing the creation of an image depicting a reality where desirous activities occur, but with the added complexity of leveraging mechanical assistance to produce something that's indistinguishable from a recording of real activity, at least to the casual or untrained observer.

(by the way, anyone thinking of dismissing these concerns with a 'think of the children' cliche - don't bother. I know numerous people who were abused and exploited in child pornography. It's not a rhetorical argument when it is something that happens to real people and which continues to impact their adult lives, and if you attempt to dismiss it on rhetorical grounds you are in effect asserting that the bad experiences of some people don't matter compared to some hypothetical liberty interest in consuming depictions of their exploitation.)

> it's not a neutral thing or a victimless crime like drug consumption

Then we ban it. We're not banning the speech; we're banning the child abuse. Take a look (figuratively, not necessarily literally) at loli/shota hentai - that's legal (at least here in the USA) because real people are not being harmed. Actual CP with actual children is banned because real people are being harmed. It's not suppression of free speech; it's protection against abuse.