User blog comment:B214/Talk About What You Want! 2/@comment-24098614-20140718200413/@comment-1359322-20140718230731

I thought i mentioned this before. But no. It won't be possible for Issei to marry Sona if he marries Rias. It may cause conflicts among the clans. Even if Rias and Sona wouldn't, but a small part of their clan might. Like during a party, and during a dance, if Issei chooses Rias, the Sitri might feel disrespected because Issei is the husband of the Clan Head, same if he chooses Sona, the Gremory might be displeased with Issei's choice. So it's not possible for Issei to marry two heiress.