Board Thread:Questions and Answers/@comment-36278278-20180721182409/@comment-29325824-20180721193602

LorialetDreamer said it best. Mamono are usually bisexual, yes, and sometimes do indulge in lesbian sex if they have nothing else to do, but because of their nature and biology(*), they all ultimatly seek a male partner for life. See them as bisexual, but also heteroromantic.

Multiple mamono have not much problems sharing a male between themselves, but ultimatly their desire for him trumps that for each other.

Now, AFAIK, KC (the author) does this not out of prejudice, but simply because this is primarily an adult hentai with a target audience of hetero males. In the Q&A, I think he said something about gay couples existing in the MGE, but that's not something he focuses on because that's not the point of his work.

(*) Considering Mamono cannot get Spirit Energy to survive from each other, only from men, and the fact that their reproductive instincts are very strong (again, something only men can satiate), it does make sense according to the lore.