User blog comment:B214/Question and Answers 4/@comment-4547267-20141026221438/@comment-1359322-20141031125118

A bit late, but i'll say something. @15sok, you know, this type of question can be considered a dumb one.

If Rias was okay with Riser, they wouldn't be a Rating Game between them. Issei and others wouldn't have gone training in the mountain, they would be weaker than Riser's team. So in the end, if Rias was okay with Riser, Volume 2 wouldn't happen. Rias would just marry Riser. Then the events after Volume 2 might have drastic changes. Akeno might not fall in love with Issei, Xenovia might not join the ORC, Kiba might become a Stray Devil and many others.

So basically you're asking a question which is literally making the story end in a very indecisive way.