"No reason why girls can't marry at 16 or 17. Some are really quite ready to do so. Some do, though I suppose their husbands are, therefore, guilty of statutory rape if they are 18 and have sex with their fiance before marriage."
All of this depends on state laws. In most states in this country, the age of consent is 16 or 17, so girls (and boys) of that age can have sex with whomever they wish, aside, in most cases, with teachers.
There are a few states with an age of consent of 18, but not a lot.
So, the Sophomore or Junior in high school, in most states can have sex whenever he or she wants to. That's the law. They can also marry.
Check the laws in your state here:
http://www.ageofconsent.com
I think that the entire attitude about underage sex -- that is, with 16-17 year olds, bears re-thinking.
Imho, it wouldn't do us ANY harm to RE-THINK our current attitudes about "teens" as "children."