Maou-sama to Kekkon shitai

One day, the Demon Queen saves Haru, a young human boy about to be sold as a slave, and takes him back to be her attendant. Some years later, the bored demon queen asks our now fully grown and handsome Haru what else she has left to do.
"Marriage," Haru replies.
Thus, a heart-pounding romcom between an innocent demon queen and her servant unfolds.