Kyo Kara Maoh! Vol 3

Kyo Kara Maoh! Vol 3 by Tomoko Konparu. A Fantasy manga rated Teen, published in English by Tokyopop in 2008.

Ordinary high schooler Yuri Shibuya is flushed down a toilet into another world, where he discovers he's the new Demon King. Now he has to rule a kingdom of demons, keep the peace with the humans, and deal with the fact that he's accidentally gotten engaged to a hot-tempered demon noble. A lighthearted fantasy comedy.