User blog comment:B214/Question and Answers/@comment-23789372-20131023004504/@comment-23789372-20131023100650

Ahh I see your right, she wouldn't want to ruin her family name, and she would want people to think very highly of the gremory family. So it makes sense the issei needs to become a higher level devil in order to have the gremory name under his belt, it's even shown when rias' mom is trying to teach issei the right way to become a sophisticated devil in volume 6 so that he can go out in public with rias without embarssing her or the family. And since her parents really like issei there only choice is to just teach him the ways of being a high class devil. Rias will always love him because he is issei of course, and she will respect the fact that issei must find his peerage so he is seen as a high class devil to people, so that the gremory family name is still intact when rias takes over the household.