wait a damn minute, my stupid ranting aside. Some people are bringing up good bloody points
Age of majority of 20-years-old.
That means that you are considered a minor if you are under that age and cannot legally give consent to being married.
There are some countries that allow adults to permit their children to marry early, but otherwise, it's non-consensual marriage. Considering that most players don't have played parents and it is against the rules to create throwaway characters (4.6), these nonexistent parents cannot legally give consent irp to the marriage. Due to one, they must not be a throwaway character without any time invested or depth, and two, they must have an adult tag as they must be old enough to be able to have kids. (4.5d)
There's no good reason why 90% of these marriages should be allowed legally, as they are in clear violation of the law without the consent of their parents. (Which is impossible to acquire unless they are played.)
Oh, and ban marriages that do get parental consent, they're just weird and shouldn't exist in such large numbers. There's a 1/16th chance that the couple would even be both 18 (with random birthdays) before the end of the final Japanese school year in late March.