This argument is a complete red herring.
First, marrying young children is not unusual. It happens all over the world, and has even occurred in this country. My guess is there are 12 year old girls all over Africa getting "married" as we speak.
As others have noted, the issue in this country is consent, not civil rights. We will let you marry any person who you choose, and who also consents. In this country, young children lack the capacity to consent. I would suggest children all over generally lack the capacity to enter into these types of contracts, but if other countries are comfortable letting children enter into contracts, who are we to argue.
That's what people keep forgetting about the difference between rights of gays to marry, and the pedophile/children, "I want to marry my dog" red herring what ifs.