Ookisugite Gomennasai!

Yuta Sato is a good Samaritan college student who offers a helping hand to Sei Tsutsui. He's stunned when Sei suddenly asks him to marry him. At first, Yuta thinks he's kidding, but the handsome quarter-Russian persists until Yuta gives in and starts dating him. After that, everything seems fine-until they decide to deepen their relationship, and Yuta realizes just how big Sei really is.

(Source: Renta!)