The justice system weights the crime of consuming child pornography by the number of images the defendent is in "possession" of. I guess laws vary, but at least in my jurisdiciton, you can be sentenced to years in prison per image. It would be easy for a single wank session to result in -- potentially -- life in prison.
The issue is confounded by widespread lack of techical acumen among judges, prosecutors, defense attorneys, and even the forensic "experts" who investigate and testify (on both sides). For example, would you consider it reasonable to charge someone with a separate crime for every copy of the exact same digial file present on a hard disk? For both an image file and an automatically-generated thumbnail image of that same file? Do you meaningfully posess a file in your browsers temporary cache? How about a file deleted from the filesystem, but still present in residual form on the disk? Some of the issues are nuanced, and it's difficult to even have an intelligent conversation about the evidence.
[source: computer nerd supporting attorneys]