User blog comment:B214/Volume 17 spoilers./@comment-117.193.172.96-20140220052656/@comment-24468758-20140220164756

The only reason why Issei and Rias are still not officially engaged is because Issei doesn't know about it, since the ceremony in the ruins of connection is at least part of the engagement. If it was just the requirement for the engagement Raiser and Rias would have had to go through it before the engagement party, since the last ones who went through it were Sirzechs and Grayfia and because of them the necessity even for high class devils is obvious since Grayfia is a high class devil from the Extra Demons.