When writing, one needs a plausible reason for one’s plot. If there’s a culture that does something particular, maybe reveres kittens for example, one needs to have a reason for such kitten reverence. It needs to be convincing enough that the reader doesn’t question it, but instead thinks, why, yes, of course they revere kittens because that’s what you do in Kittenlandia. Maybe a beloved child-queen Myleigh adored kittens but died of childhood leukemia and her last wish was that kittens would be sacred from that day forth. Or, maybe the goddess Nephreini decreed that her sacred animal was the kitten and you could tell her devotees by their scratches. Whatever the reason, the fact that kittens are revered becomes as obvious as the fact that the sky is blue and just as unquestioned.
When Rachel and I wrote Sealed by Fire, we needed a plausible reason why male relationships would be common and accepted in the subculture we created. Rachel settled on the lamia myth, which is about a half-woman, half-snake creature. There are no myths of male lamiae that we could find, so we decided that lamiae had to come from somewhere; what if male lamiae weren’t half-and-half creatures but all one, and all the other? Meaning, they were either snake, or man; never both. Only the women had the snake-person form. Furthermore, the two groups kept separate from one another except at breeding time, so they lived in gender-segregated communities by tradition.