So, the MC slept with the ML initially as a replacement to ML's brother? That's so shitty ...

I mean, they only slept together twice right (before they started dating for real)? Both times drunk, first time was a misunderstanding. I wouldn't say he slept with him to use him as a replacement, they both just got caught up in the moment and found comfort in each other since they had no one else to confide in. Sarang was also in love with someone else at the time.