Comment by Jesse Miller on Grandpa's condom

@Grandpa's condom there is no reincarnation. Dongha was pulled into a novel written by Sungwoo (his first love and the guy he kissed at school, who left after someone filmed them together), Dongha didn't want his sexuality to come out so Sungwoo didn't out him either and accepted his fate, basically. 5 years later Dongha received a novel written by Sungwoo and got hit by a truck that put him in a coma. That's when he was pulled into the novel and met Yujin (who is the novel version of Sungwoo). The thing about Yujin is that his biological name is Yu Sungwu. The end where Yujin gets stabbed, he pushed Dongha back into the REAL WORLD and he awakens from the coma. It's obvious that him and Sungwoo have made things right between them and are dating. The comment about his name being Yujin is basically a petname of sorts his parents called him when he was little. 

Yujin (novel version of Sungwoo and birthname is Yu Sungwoo)
Yu Sungwoo (real life first love of Dongha and whose petname is Yujin)

Hope this all clears it up for you