Ryuujin no Hanayome: Mi ni Furu Ai de Shitodo ni Nurete

That's such a pretty face you're making... Did you get that wet just from a kiss? I can feel something building up inside me... My mind is going blank...! Born in an age where twins are deemed unlucky, Miya is raised as a household servant, a position far below that of her elder sister, Mina. When a wealthy man takes a liking to Mina, Miya finds herself married off to a man she has never met in her sister's stead. Nervous at first, the warm words of her suspiciously handsome husband open her heart, and not long after, she gives in with her body as well... "I never knew that a person's skin could feel so warm." Thus begins the pure-hearted tale of mystical, matrimonial adoration, as Miya comes to know the love of her husband. (Source: MangaPlaza)