Claymore

Yoma (humanoid shape-shifters that feed on humans) and Claymore warriors (human-Yoma hybrids that kill Yoma for a fee) alike are powered by a demonic energy, Yoki, which allows shape-shifting and extreme strength. When warriors use too much Yoki, they "awaken," becoming a super-Yoma called an Awakened Being. The act of awakening is likened to the feeling of sexual climax, so while both male and female warriors existed in the past only the women proved to be successful warriors and so the creation of male Claymores stopped altogether.