User blog comment:Crimson and Darkness/Shin DxD 3 discussion/@comment-176.54.22.178-20200130133402/@comment-10330822-20200131043247

To be honest, I understand the arguement for why they don't have harems, but it still doesn't really make sense. If the devils are having a population issue and a power issue because of the war, why not have the strongest try and produce as many children as possible? If devil women are so hard to impregnate, it just makes more sense if you want to strengthen the species as a whole.

Also, there's no reason someone couldn't overturn that rule. Especially if one of the girls make it their wish to ensure Issei can still be a Maou and marry all of them.