Majuu Jouka Shoujo Utea

Mitsuki and Karen are two ordinary High School girls, until they're recruited one day by a mysterious creature to become Magical Girls and fight evil. To their dismay, they find out this actually means having sex with monsters to subdue their influence on society. They start the story being manhandled by the beasts over and over, before eventually giving into the pleasure and becoming "beasts" themselves - capable of growing sex organs of the opposite sex and using them to sleep with other girls of their choosing so the cycle may continue.