User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-116.75.184.241-20150118113637/@comment-16281343-20150118172807

He didn't stop Rias. He stopped Issei from declaring his love for Rias in front of him. He is okay with love transcending race, so no need to worry of someone trying to break their relationship.

He said to Issei something like this after Rias declared her love:

"I know you have something to say, but don't say it now. You should be a high-class Devil first if you want something to say. Once you're promoted, you and Rias can go to my territory and I will serve you the apples that is the specialty of the Bael territory. I don't hate you because you defeated Sairaorg. Rather you did a good job. However, I'm still an old Devil, by now I do not want to pursue change."

Basically, old dudes sticking to tradition.