User blog comment:B214/Question and Answers/@comment-94.128.38.124-20130624153153/@comment-5185939-20130716235246

Rias's mother already said Rias needs to be willing to let Issei have him harem.

She also commented that if Sirzechs hadn't been so busy being Lucifer, he'd probably have had to marry more women than just Grayfia.