The survivors of humanily are thevalkyries, except me

Sen'otome-tachi to Kizuku Moto-Teihen Murabito no Saiky Hremu: Smannin no Onna ni Taishi, Otoko wa Ore Hitori / "The only ones left alive are us, the Valkyries." Oswald, a boy who had been scorned for his unusual magical power, awakens to find himself in a world several centuries after it was nearly destroyed by an evil g**. It turns out that the sexual arousal triggered by Oswald's magic is the key to empowering the Valkyries. In other words it becomes a great power capable of saving the world!!