User blog comment:B214/Volume 22 spoilers/@comment-27083298-20160610145130/@comment-1359322-20160611004658

That was the agreement between 2 clans Gremory and the Phenex it has nothing to do with the Devils or anything. Rias' father and Riser himself wanted Rias to marry earlier. Rias herself never wanted it.

Also normal for a girl of Rias' age to get married and have a child? Devils live for over a hundred thousand years and Pure Blood Devils especially have low birth rate, how can anyone expect Rias to have a child shortly after marriage? Sirzechs and Grayfia took 200+ years before baby Millicas was even born.