Kyojinzoku no Hanayome

Kouichi Mizuki has just graduated high school and is looking forward to the future, but after his last day at school, he finds himself summoned to Eustil, a world of titans! His summoner, Prince Caius, has brought him to the kingdom of Tildant to become his bride and fulfill a prophecy, but their considerable size difference makes Kouichi worry that he might not survive being penetrated by his huge new "husband". Caius agrees to send Kouichi home after a month if he is truly unhappy; yet as time goes by, Kouichi finds himself getting used to being a "dwarf" in a land of giants. And in spite of his protests, Caius's nightly ministrations on his body fill him with unimaginable pleasure... Source: anime planet