User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-65.8.85.249-20140620230159/@comment-82.48.188.251-20140620230928

Issei would never become a Maou since it would mean "No harem", like Venelana said to Rias (Sirzechs has only Grayfia because he's a Maou).