Aku No Kanbu To Amaama Na Hibi summary:

Every inch of my body within the sworn enemy's tender grasp... How did this happen?! Gangly hero meets monstrous yet charmingly sweet demon in this guiltily sweet romance. -- I'm Masakiyo Kato (21), also known as Skull Black. I may be somewhat scrawny, but I'm a protector of justice. Today, once again, I'm defending the town against Baphomet, a commander from an evil organization. At least that was the plan! He somehow ends up pinning me against the wall... again. As if that weren't enough, he shows up at my place asking, "What do you want first? A bath? Dinner? Or do you want me?" What the hell does this giant beast want? And why is he trying to get me to marry him?