Boukun Ou no Kekkon

It'll be simple once we've bed each other. You'll lose yourself to me... especially if this is your first. Ritsu, a normal college student, receives an unexpected letter. It says that he is betrothed to a king in a foreign country per his late grandfather's promise...! With no interest in love, let alone marrying a man, he refuses! Ritsu travels far to arrive at the royal palace where he meets Seren, a king with unparalleled beauty. Seren steals a kiss, claiming there isn't a living soul who hasn't fallen for me...

(Source: Mobile Media Research/Funguild)