Houtou Ouji to Toraware no Kemono

Alan, the second prince of Corus, asks Sauro, a warrior of Matara who is being held in a dungeon, to help him escape in order to stop the civil war that was planned by his mother, the mistress of the king, to usurp the throne. Saul asks Alan for a "vow" as a condition of his cooperation, but that "vow" is to connect bodies and declare that "I will embrace you from now on"! ?

After escaping from the country, the two deepen their understanding of each other on the way, but it turns out that Saul's "vow" was an act of love and Alan became his fiancé...! A dramatic romantic romance in which Alan learns the joy of being loved while being embraced by Sauro

(Source: Renta!, translated)

Note: Chapter count includes an extra.