User blog comment:B214/Volume 23 discussion blog/@comment-185.112.105.215-20170419115644/@comment-31137559-20170421051259

And a pureblood high-class devil from noble families could only be married to each other to maintain proper relations to each other. Yet Issei is engaged to Rias and it is accepted across the Underworld. Not because the Gremorys have Millicas and he's the back-up. But because Issei has the strength to rise up in the Underworld and be engaged to a pureblood devil from a noble family. For a reincarnated devil to be married to a pureblood devil would be blasphemy to them and yet it's going to happen. The devils in the modern society value and acknowledge strength over their pride. Lord Bael can't do anything against Sairaorg being the heir despite him being the head of the highest ranked household in the Underworld. Do you think they would stop Issei and Rias' son from being heir based on something that was said in Volume 2 that doesn't hold much weight much later in the series seeing that Issei and Rias are going to be married and she is going to be the head of Gremory. The other higher-ups would probably just write it off as a Gremory matter and stay out of it. But the Gremory's status isn't going to change.

It's very hard to make comparisons to DxD, but I'll say this. In Code Geass, Lelouch was the 11th prince and 17th heir of the Britannian Empire despite his mother being a commoner from birth. Now she did rise up in rank but Lelouch's mother wasn't nobility. And then she became an Empress of Britannia and one of the Emperor's consorts and there was a lot of envy from other higher-ups and the other consorts due to her not being a noble. Yet Lelouch and Nunnally were still members of the Royal Family and were heirs to the throne.

The higher-ups will not try to force their views on the Gremory family about purebloods. Lest they turn public opinion against them. If Issei who isn't a devil by birth and can become a Gremory and marry Rias and become a noble, then I see no reason why their son wouldn't be eligible for heir.