Ryuu no Yuigon

Asagi grew up on his mother's stories of Lord Tenran, the beautiful, noble demon king of the east. His dragon lineage means that Asagi will one day serve Lord Tenran as his mother did, but when that day finally comes, he's in for a rude awakening: Lord Tenran is not only egotistical and vulgar, but also happens to be a total uke who loves teasing Asagi about getting him in bed!

(Source: Storm in Heaven)