User blog comment:B214/Talk About What You Want! 2/@comment-108.27.243.235-20140707032420/@comment-1359322-20140707104412

It's not like Rias accepted the girls from the start too.

Also you're misunderstanding something, Riser never had any intention of making his servants his mistresses. He only likes to have fun with them but there was never any indications of sort that Riser wants to marry them at all.

Well that may be why Rias dislike the idea to marry Riser, from a female's POV, she probably though Riser wasn't going to be serious in loving her because of his relationship with his servants.