User blog comment:B214/Volume 17 spoilers./@comment-121.54.54.40-20140515155159/@comment-1359322-20140517235348

Either way, Issei isn't really engaged to anyone yet. I know many assumed that Issei is engaged to Rias but he isn't, the ceremony in Volume 8 is just to test on 2 things only. Issei's suitability as the husband to the Head of the Gremory Clan and his and Rias' feelings towards each other.