User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170331222853/@comment-24098614-20170331233807

This has been obviously stated in the first volume of the series when it was stated that Rias's father has a large Harem even bigger than Issei's, and he makes no qualms in hiding it either but their very nature Polygamy is normal to them because Devils are ruled by desires and ambitions so it makes perfect sense that Polygamy is normal to them.