This will be TL;DR
I am not sure if they are following the base story of the Japanese Creation Folklore, but you will see here that it is only Izanagi present in that realm and Izanami (the female counterpart) is no where in sight (only angels are there). Here is theory where Izanami might be:
If I remember Japanese Creation Folklore correctly (from the Noh), Izanagi and Izanami created the world through union/coitus (seggs) and after a while Izanami after giving birth to their creations lost her "light" so to speak and her very existence began to deteriorate. She technically died leaving Izanagi's realm and her soul descended to he 'mato' or the underworld (you can call it tartarus to paint a clearer picture). Izanagi tried to retrieve her soul but when he went there, Izanami is already deformed and Izanagi in shame (she can't turn her back to her usual self) had to escape from the mato, being pursued by the other monsters (probably created by Izanami's monster self) in which Izanagi had to create the very first 'peaches' as a weapon to keep them at bay (like projectile granades). When he resurfaced from the mato, he bathed and from the waters came the 1st 3 major deities, Amaterasu (from one of his eyes), Susanoo (from his nose) and Tsukuyomi (from the other eye).
I think this same folklore is also the basis for Mato Seihei no Slave, where only females (i.e. representations of Izanami) can receive powers from the Mato Peaches (peaches created by Izanagi). And that is why most of the strongest enemies in that manga are mostly females as well.
