Binetsu Kakumei

Tatsuki and Ohta are two beings from a supernatural, mythical realm, masquerading as "cousins" in the human world. Tatsuki is in fact the successor to Ryu-Oh, the dragon king, and possesses the power of the Nyoi-Houshu, a magical dagger. Ohta is a Zashiki Warashi, a protective house-spirit without a home, and is also Tatsuki's subservient lover. When their human classmate, Kouhei Misaki, accidentally witnesses the two having sex, he mistakenly believes that Tatsuki is forcing the quiet Ohta into it, and he resolves to break the two apart.

(Source: Juné Manga)