The Survivors of Humanity are the Valkyries, Except Me

"...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 god. 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!!