18+ Youjuu Kyoushitsu Gakuen

Earth is being invaded by a race of unclean beasts who wish to take over the planet. However, since Earth's environment is hostile to the demons they hatch a plan to impregnate as many human females as possible in order to create a race of human/demon hybrids which will rule the Earth by proxy. But the "Interplanetary Mutual Observation Agency" sends a duo of enforcers to Earth to hunt down and destroy the alien race they call the ‘demon beasts’.