Here's an idea for the human semen cattle fetish in relation to elves, using hentai settings as a base:
>Known as the Fae, Elves, Fairies, Fair Folk, Dryads etc.
>Elves are essentially the gardeners and protectors of World Trees, who created them for such purposes
>They are only women, but are physically weak, around even with men
>Their protection methods involve hidden settlements, traps, glamours and other in-direct defenses
>However, once these defenses are bypassed, elves are pretty defenceless against other warrior women, and would often get enslaved
>Enslaved elves were used as servants, entertainers, and even sex slaves
>Despite not being born sexually, they are capable of sexual reproduction, and would get impregnated by human men
>Unlike elves born from World Trees, elves born of an elf are not defenceless, they can take on human women in even combat
>These elf born elves would then lead slave revolts, liberating their tree born elves and kidnapping several men to perpetuate themselves
>Will regularly raid humans or extract tribute of males, in practical terms because they need to in order to have proper defenders, but also because sex is pretty fun