"parents might reasonably choose to tolerate sexting while warning their teenagers against it"
UH no, that would be "reasonably [intentionally] choos[ing]" to perpetuate child pornography.

Kiddie porn is illegal, and society has decided it is comfortable with strict liability for violations of age limitations on sex (that means no intent is needed). This law requires a threshold of actual intent, so it is much more lenient that punish a guy for being lied to by a 17 and 1/2 year-old. No jury is going to convict a parent who did not know anyway. I don't in general approve of mandatory minimum sentences though.