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First and foremost, if you're disgusted by incest vibes in fiction then this one isn't for you. They aren't blood related, but grew up calling each other brothers because they only had each other. Seungdo saved Yegeon from the orphanage director and raised him himself. Same with rape, it's pretty plentiful in this... But there IS a story under there and it's sad...
Seungdo (MC) is an orphan who's raised his brother Yegeon (ML) since they were both in the orphanage. The director of the orphanage was a pedophile, Seungdo protected Yegeon from him by putting himself in the way. When the director started looking at Yegeon he took Yegeon and ran away from the orphanage. Eventually Seungdo met this bar owner guy, real piece of shit, that tried to get him into prostitution and drugs. Seungdo was really young at the time, basically an impressionable teenager. The bar owner was attracted to him and wanted to trick him into sex. He told the guy he wouldn't work there at first, but then Yegeon needed tuition, so Seungdo agreed to "work" there (prostitution) and let the bar owner take his virginity and train his body for sex with men. He got Seungdo addicted to drugs so he'd stay there too.
Backstory out of the way. By the time the story starts, Seungdo has already stopped working at the bar, though he is still sleeping with men for money. The story opens with him having sex with a man for money. He gets a text from the bar owner, who is trying to get him to come back, he tells him that he has the drugs and Seungdo agrees to do one more job for the drugs. Turns out that the client was actually Yegeon, who was trying to see if his brother was still working at the bar. He knows that Seungdo has been whoring himself out for his sake since they were young and he's trying to catch him in the act so he can bring him back.
Seungdo has MAJOR self esteem issues after having whored himself out and given his tattoos/general way of life. He wants Yegeon to have a normal life, but feels like he's a stain on Yegeon's life, so when he's caught he decides to try and leave his life right then and there. He tells Yegeon to leave him alone and that he's living like that because he wants to. That he likes drugs and sleeping with men, he wants Yegeon to forget him and leave him behind. Yegeon doesn't know what to do, so he says "then I'll do it" and ravishes him while Seungdo protests and ends up giving him higher quality drugs himself so he won't go back to the bar. This starts their sexual relationship. There's a misunderstanding on Seungdo's part that persists for a bit that he has corrupted Yegeon and that's why he's suddenly gay and giving him drugs. He thinks, because he's a stain on his life, he has corrupted him. He eventually realizes that Yegeon just cares for him the same way he cares for Yegeon, that's why he took on the sex and drugs role, since Seungdo was saying he wants that.
The romance aspect is a bit confusing. It's hard to say if they had feelings for each other beforehand or if they just ended up in a sexual relationship because of what Seungdo said to Yegeon when he got caught working at the bar. It's implied that Yegeon at least had feelings, because he says something along the lines of that "I used to hate it when it was said that we are strangers, that we don't share blood, but now I'm actually glad it's the case because I can stop acting like your little brother". He's now glad that they aren't related because they can be together without that being an issue. It's also said that Yegeon wanted to always be together with Seungdo, and that's said in a context where marriage is being compared. Seungdo says that Yegeon feels the same as him, which could mean Seungdo also had feelings for him. Actually that would explain the pang he feels when he sees Yegeon talking with a woman at work.
Seems like they were "brothers" because they lived together and took care of one another, but eventually it became more for both of them, but they were too scared to break the status quo/really look at that head on until things were broken.