User blog comment:B214/Volume 22 spoilers/@comment-73.46.6.122-20160824195231/@comment-9595671-20160824225630

I agree, plus knowing Rias, she'd want to limit how many girls Issei gets in his harem and knows girls in either of their peerages will likely end up that way.

If Valerie's her official new bishop, then that's one less worry because I think we can all agree Valerie likely has a crush on Gasper. This volume seems to hint Kiba/Tosca pretty well too.