Mugen no Kyoukai / The Edge Between Dream and Reality

 Hentai is based on the visual novel of the same name by studio EVE.

 After waking up, Kengo-kun found himself lying on the shore of a strange, unknown lake. He surveyed the surrounding area - green foliage, a quiet water surface, and an unusually bright sun... But away from all this unearthly beauty, alarming cries of strange creatures were heard. Kengo went towards the cries and found catgirls having sex with monsters... Something is wrong here, thought our hero, while examining his body - he was wearing steel knight armor, and his body itself had become twice as large. Thus, an ordinary otaku ended up in another world and became a prince named Aoteru - known as a demon fighter.