The Titan's Bride

Kouichi Mizuki is about to take his final exams and graduate from high school, when he suddenly finds himself in a world of giants, the titans. He meets the prince Caius who announces to Kouichi that he has become her husband! It is a prophecy speaking of end of the world which pushed the prince to take as wife, an earthling.
Caius does not intend to send Kouichi back to earth, so Kouichi must respect his choice.