User blog comment:B214/Question and Answers/@comment-37.231.49.206-20140110200545/@comment-1359322-20140111031834

Actually there's also the issues with the clans. Both Rias and Sona are the next Head of their clans, so the husbands they marry are going to be considered as the Head of the clan too along with Rias and Sona. So here's the issue: If both of them marries Issei, who is supposed to be the husband of the clan head. How is Issei going to introduce himself, Issei Gremory, the Sitri wouldn't be pleased, Issei Sitri, the Gremory won't be pleased. Issei Gremory Sitri, the Sitri won't like the fact their name are placed behind the Gremory. Issei Sitri Gremory, the Gremory won't be pleased with their name placed behind the Sitri too.