A hentai focusing on monster girls.
Long ago, the Twelfth Demon Lord was usurped by a succubus, who infused her essence into all monsters in an effort to end the eternal war between man and monster, causing them to feminize, anthropomorphize, and become incredibly laviscous. This had the unfortunate side-effect of causing them to become corruptive as well, mutating men who have sex with monsters into incubi over time, and instantly converting any woman who has sex with a monster into another monster of the same type. Monsters can only bear other monsters of the same type, (who are, of course, female), and incubi, the only male monster, can't bear children for obvious reasons. This is quite the problem for sustaining the population...