Shokku: Gakuen Youshokutan

Aramaki is asleep to refill power, used up for a series of battles. However, it is suddenly interrupted by Asuka, an exorcist. He thinks of obtaining her power, but Asuka is very strong. So he decides to transform himself into a human and approaches her. As a result, they become boyfriend and girlfriend. He still waits for a chance to embed the seed of monster into her body. Once embedded, she will never escape from its pleasure.

(Source: ErogeShop)