Scarlet

College student Akio is dating his beautiful level-mate, Ryou. But Ryou is unable to withstand the slightest loneliness, and every time a girl approaches him, he’s unable to decline and goes out with them. One day, Akio was introduced to a girl that Ryou was considering marrying, and that made him decide to break up with Ryou totally.
(from Blissful Sin)