I'd be suspicious of any two 15 year olds getting married within a religious group as it sounds like an arranged marriage. Good reason to opppose the legality of underage marriages.
The problem is that...
a. Not all marriages of this type (especially when you get into 16-17 year olds) are arranged, or at least not without the consent of the partners;
b. You'd have to, at minimum, recognize marriages from other countries, or else you'd have problems with immigrants. In which case, your arranged-marriage fiends would simply fly the kids of to Vzrglstan to get married, then fly them back.