In nearly every state in the USA, it’s perfectly okay for children as young as 15 to get married. In New Hampshire, that child can be 13. In South Carolina, 14, only requiring parental consent. In Florida, not only can minors wed, but minors who have been married before don’t need parents’ permission, meaning a 13 year old wed in New Hampshire can get divorced, move to Florida, and marry again at any time under 18 without parental consent.
Many of these states, coincidentally, allow people below the minimum age to get married with certain qualifications met, usually getting permission from the court. In other words, the state gets to decide if children barely in their teens get married or not. The usual cause for such a request, of course, is if a teen gets pregnant.
Not a single state requires that everyone getting married be 18, without exception.
You can see for yourself in this excellent series of charts about marriage laws in the USA.
So, let me get this straight: religious conservatives scream about girls under 18 being able to get an abortion without parental consent, but children getting married is just fine?! And these same religious conservatives scream about gay marriage, but aren't doing anything about the horrific, barbaric practice of child marriage in the USA?!
Atheists, secularists, and agnostics are a very diverse group, but I can say with confidence that every single one of them that I know does not support child marriage, under any circumstances, and many are actively campaigning to stop the practice.
As usual, actions speak louder than words.