Here's one example. Let's say there's a 16-year old girl and a 17-year old boy who are in a consensual dating relationship and, you know, take some racy photos of one other. The photos were taken consensually, not shared with anyone else, and stored only on their own computers/accounts. And let's say they're living in Florida and under state law, they're legally old enough to have sex with each other.
It would be fear-mongering to expect that these two happy teenagers would ever be prosecuted and convicted on "child pornography" charges, right? Except they were. And, as I wrote in 2007, a Florida appeals court upheld their criminal conviction: http://news.cnet.com/2100-1030_3-6157857.html
Yes, sorry, Timmy and Mark, sometimes a judge and jury will let it go that far. Sometimes they will "literally lock up your children..."