On this subject, there several separate things that are treated as (almost) equivalent by a lot of laws:
- recording children in sexual positions
- recording teenagers in sexual positions
- creating virtual images of that look like children having sex
I doubt anyone will disagree #1 is bad. Treating #2 as equivalent to #1 (usually through law that define "child" as anyone under 18, regardless of the age of consent) results in ruining teenagers' lives for no real gain (and that goes doubly in the cases of teenagers being charged for creating child pornography of themselves). Treating #3 as equivalent to #1 is punishing sexual deviance (thoughtcrime) rather than actual acts. If someone is excited by children having sex, I'd rather they jerked off to virtual pictures rather than have go around frustrated, because that'd make them less likely to actually molest children (bear in mind that the availability of porn is correlated with lower sex crimes rates).