Ulji Maseyo, Seonbae-nim!

Eunsoo Seo's love life was always in turmoil. People always used him to get what they want, but not one thought about what Eunsoo wanted. When Eunsoo was about to be abused by his boy friend, a stranger named Hajoon Kim swoops in to save the day. Saved by the stranger, Eunsoo began his complicated relationship with Hajoon... literally, "Day" and "Night".
(Source: Pocket Comics)
Note: Chapter count includes five side stories and three extras.