Choukyoushi

From JShoujo Scans:

Sawa has been going out with Manabu for almost a year. He's kind, gentle, and giving. She has no complaints, seeing as how he's the perfect gentleman, and she figures that if they continue to date, they'll eventually end up marrying. One day, Manabu takes her out to a bar, where he introduces her to a friend of his, who turns out to be someone she used to know back in college. On her taxi ride back home, she finds that she took her former classmate's cellphone by mistake. He asks if he can come pick it up at her apartment, and when he arrives...