Demon King is a shy girl(no dicks), scared of men, who is saving her virginity for The One, the heroic prince-like hero who'd take her heart and marry her and then they'd offer each other's virginity in their first night.I imagine the demon king like a big dominant milf (with a cock), but she could be a loli, that would be a good plot twist
That's why only a virgin can "defeat" her.
That's why monsters aim at fucking you silly, to corrupt you away from her