If I understood recent stories in the news correctly, there is not 1 state that bans underage marriage. You can still do that with the parents permission, some states as young as 13. Now it may be more for the fact that the states simply never updated the laws. I should be a minimum of 18 in all states.
https://www.google.com/#q=Legal+age+for+...in+America
https://www.google.com/#q=Legal+age+for+...in+America