Negai wo Kanaete Moraou to Akuma wo Shoukan Shita kedo, Kawaikatta node Kekkon Shimashita: Akuma no Niizuma

Mercenary adventurer Theo Aspel is tragically weak, leaving him no choice but to repeatedly joins various parties—only to get kicked out. In order to get stronger, he succeeds at summoning a demon that will grant him a wish. But the minute he sees her, his wish to get stronger flies straight out the window.
This devil, who hadn’t been summoned in many years declares: "I will grant your wish, but you must give me something precious to you." Theo stutters out a reply: “W—will you marry me?” The devil is stunned to say the least, but grants his wish, making them husband and wife. They embark on a happy married life together, causing quite a stir around them along the way. This is a story of a boy and a girl that's a just a tiny smidge dirty.