User blog comment:B214/Volume 22 spoilers/@comment-1359322-20160724163251/@comment-1359322-20160724165359

I wouldn't really say Sirzechs is wasting time on making children more likely he wasn't lucky to conceive one with Grayfia. Devils have low birth rate especially Pure-Blood Devils. In a sense, Rias and Ravel having one in such a short period of time is considered lucky and rare.

Or maybe Ravel's hidden potential is to help boost the child birth rate of her family.