I have checked the legality and in many places, including Nevada, a girl who is 16+ can marry if one of her parents gives permission.
Now, if you are well over 20 and are a guy, would you do it?
Pros -- great sex, someone who will look up to you and dote on you, some whose body won't deteriorate for a while, someone you can mold into the ideal mate and person
Cons -- people might stare, you could be called a child molester/cradle robber, she might start liking guys her age, immaturity
The assumption is that she knows what she is getting into and is doing it voluntarily. Also she signs a pre-nupitual agreement so she gives up all rights to support and community property.