User blog comment:B214/Volume 22 spoilers/@comment-201.215.165.7-20170213165134/@comment-31137559-20170215021740

@B214 Then the whole reincarnation system would be pointless. What's the point of being in the devils'society and being able to rise up in the ranks if their children would be considered half-blood if they had children with another devil? It doesn't matter if you're a reincarnated devil or the person's origins or whether they are another species in addition to being a devil, you are a devil to the devil society and they will see you as such. If Issei was never reincarnated as a devil, you have a case. Vali, Lilitifa Vepar, Liban Crocell, Misteeta Sabnock, Coriana Andrealphus, and Ladora Bune. What do they have in common? Only one of their parents was a devil. And their other parent wasn't a devil in any way, shape, or form.

And well they won't have as pure a devil blood as someone who was born from two pureblood devils. But if Rias' and Issei' children were going to be considered half-blood then it would have been noted. It's just that there are sevils that want as pure a blood as possible and feel that only pureblood high-class devils can marry pureblood high-class devils.