The Titan's Bride Vol. 1 (manga)

Senior Kouichi Mizuki is about to take his final exams and graduate from high school, when he suddenly finds himself in a world of titans. Not only is he no longer anywhere near home, but one of these giants, Prince Caius, has claimed Kouichi as his bride! Thanks to a disturbing, world-ending prophecy, the prince’s upcoming nuptials cannot be with anyone of his world. Since Caius has no plans to send Kouichi back to earth, he is wholeheartedly set on taking Kouichi as his “wife”!