I think there is a good case for making the sale of child porn illegal and certainly the creation of child porn (by molesting actual children) should be heavily criminalized¹. However, the argument for criminalizing mere possession of child porn seems less strong. In particular I can think of a bunch of reasons off the top of my head why criminalizing child porn would increase the number of molested children. Let me summarize a few here.

1. There are documented problems with the harshness of child abuse penalties (lifelong sex offender status etc..) discouraging family members from turning in their relatives. No doubt people are going to be an order of magnitude more reluctant to send their uncle or brother to jail for merely possessing child porn (whatever the truth is they will tell themselves he doesn’t hurt anyone). If child porn was decriminalized it might encourage more people to report pedophiles they know to social services who can then provide treatment.
2. Making possession and sharing of child porn illegal radically restricts the reuse of child porn. Suppose there is X amount of child porn that has ever been made and X is more than enough to satisfy most people. If they could all access that X amount there wouldn’t be as much motivation to molest kids to make child porn. On the other hand by making it illegal we likely take a great deal of that child porn off of the internet and balkanize the rest of it encouraging more people to pay for child porn and thereby encourage others to make it.
3. Prohibiting possession of something always creates a criminal element who tries to deliver that thing. Organized crime creating child porn may harm children more than the alternative. It’s hard to rate relative badness of this sort of thing but being repeatedly molested by a criminal enterprise may be much worse than being occasionally molested by your uncle.
4. Whenever you criminalize an activity that a fair number of the perpetrators see as victimless (”I just downloaded the pics I didn’t hurt anyone”) you reduce the barrier to further crime. This is the same way that government prohibition of weed has made it a gateway drug. In the legal scheme where mere possession of child porn is a felony many people may not see it as a big leap to go from mere possession to purchasing child porn from sketchy eastern European websites. On the other hand if you make it a felony to purchase child porn but not merely to possess it you might very well do more to suppress the profit motive in abusing children.
5. I suspect that some people who like to look at child porn (or at least who would were it not a felony) are actually decent people who do believe that children shouldn’t be molested, they just happen to be turned on by it. For instance some girls both feel very strongly about protecting college girls from being taken advantage of yet also find porn that depicts it really hot. I have no reason to believe that there aren’t some people who are turned on by child porn who feel similarly. By decriminalizing mere possession you can enlist the aid of these people in tracking down the abusers who make the material.
6. Making possession of child porn a felony encourages people to be very careful about who they share it with reducing the distribution of any single image. This makes it much less likely that the child pornographer will be caught since it is less likely to make it to the police in a timely fashion.
7. Sending otherwise productive members of society to prison and especially inflicting punitive sex predator laws on them is not only a huge societal cost but makes them more likely to offend. The guy who is turned on by molesting kids but has a nice career, friends and family may very well keep his desires under control. Send him to prison for a few years because you find some child porn on his hard drive and he may very well become bitter and having little left to lose descend to actually molesting children.
8. Maybe people who would otherwise get treatment are scared of asking for help lest their child porn collections be suspected and they get sent to jail. Since therapy seems to be helpful in preventing more abuse this could be another potential avenue of harm.
9. Perhaps the release provided to pedophiles by looking at child porn makes them less likely to go out and molest. While I think this is the least likely of all these points it needs serious study not another mass media sensationalization about a correlation between the two (duh people who want to look at child porn more likely to molest kids).

Now maybe these factors all end up getting balanced out by other ones but I seriously doubt that these questions have been seriously studied. They certainly weren’t why we passed the laws against child porn. Frankly I don’t really know whether child porn laws help or hurt and given that I’m unsure I think the default state should be not to regulate speech/content. However, one can reasonably disagree with this point.

What one can’t reasonably disagree with is that we have a morally responsibility to seriously examine whether laws criminalizing child porn reduce or increase the number of children who are molested. If you are in a position to seriously influence the law (politician etc..) and you allow more children to be molested because you want to make something as disgusting as child porn illegal or just because you are too lazy to consider the arguments then your not much better (maybe worse) than a child molester yourself. At least child molesters have the small excuse of extreme psychological pressure but if you let it happen out of mere laziness you have no such excuse.

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1. By child porn I mean photographic evidence of what would be otherwise termed molestation of a child. Teens taking pics of each other or people taking candid shots of their kids getting out of their bath even if later used for sexual purpose are not what I am talking about here. Maybe other laws should cover these situations but that isn’t the subject here, nor will I address here the question of where to draw various age lines. ↩