I think that you're taking this to an extreme.
Most states have Romeo and Juliet clauses, allowing minors to have sex with those who are no more than four years older. Many states have an age of consent of sixteen or seventeen (RESEARCH- do it). Most states have done away with sodomy laws.
Now, there's one thing about everything you've said: the government can only intervene if they know about it. If an underage girl sends a guy a naked picture of herself, no one will know about it until that picture gets sent to everyone at their school, posted on the internet, or otherwise brought to the attention of authorities. All of the problems associated with sexting are problems with problems becoming un-private, which is, to some extent, your own damn fault. I'm seventeen, and I've yet to take a nude picture of myself, much less send it to anyone untrustworthy. Maybe I'm the exception to the rule, but, either way, not everyone is doing it.
Underage sex is the same way- unless a couple is stupid enough to record it or unfortunate enough to get caught, they can't be prosecuted. There are easy ways to avoid either. There are unfortunate couples that are unjustly punished, yes, but not most couples.
Laws are changing, and are easy to avoid. It's like speeding- most people do it, knowing that there's a very small risk that they'll be caught, as long as they're sensible. Don't speed in front of cops; don't have sex in front of parents or cameras. BAM, SOLVED. I'm all for the age of a lot of things being lowered to sixteen, and I'm all for Romeo and Juliet clauses- however, there's just a whole lot of sensationalism for very few arrests.
