User blog comment:B214/High School DxD EX/@comment-117.195.118.24-20150629125656/@comment-24502975-20150629163419

Most likely this story takes place after the anime to avoid spoiling LN events for fans who buy the BDs. In which case, Ravel, Rossweisse, Le Fay, Kunou etc. haven't joined the harem yet. Kuroka's likely the exception because she hinted (understatement of the century, I know) at wanting his kids in the anime, but you don't know if she actually joined the harem or not.