Oujisama no Hanryo wa Batsuichi Around 40

Tatsumi Yagasaki, a corporate writer who is tired of life, is forced to help Prince Nico of Rasheed Kingdom, who came to Japan to produce a guide book about it. Though being dragged around by the Prince everyday exhausted Tatsumi, he was touched by seeing Nico's honest and innocent smile. Before he knew it, they drunkenly embraced each other, and Tatsumi woke up the next day to realize that they did it. Tatsumi was afraid that he'd be punished to death as a consequence, but he saw that Nico was happy and embarrassed at the same time. Due to an old custom that dictates that one must spend their life with the one they gave their virginity to, Tatsumi has no choice but to become Nico's partner!

(Source: Houbunsha, translated)