My High School Sweetheart, Mukahi

Aoi, landlord of a share house and a florist, can't keep his mind off his first love, Mukahi. One day, the two bump into each other at a wedding. They realize that they are still in love with one another, and spend a blissful night full of passion. However, Mukahi comes to live at Aoi's share house, and there's no doubt about it... Mukahi is a high school senior?!

(Source: MangaPlaza)

Note: Includes one extra chapter.