User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-85.154.146.101-20140209163918/@comment-67.176.158.104-20140210013157

Maybe Riser wasn't really what he appeared to be when he was in front of Rias' parents. He might have been only pretending in order for them to approve of him as Rias' groom. And acted in a more disrespectful way when they were not around. Maybe seeing his clan as superior to Rias' only wanting her out of lust and so she could have his kids. Rias could have known this and that could have contributed to why she refused to marry him.

And Sirzechs could have known as well or had recently found out and that could be why he decided to help Issei stop the marriage.