Cheating? Why cheating?
I mean obv not cheating cus mc is SA'ed here but then the author will make mc somehow dumb and fuck his brother cus his s/o ain't giving him enough attention so therefore that's where "cheating" comes in. Typical younger brother is "better" trope of a plot
Oh it's not his significant other. He has a one sided crush and thinks he finally got to sleep with him but turns out it was the brother.
Here's the description : One drunken night, I thought I finally slept with Ko Woonha, the person I’ve quietly loved for years. At least… that’s what I believed.
Dang it I'm not sure cus it's the first chapter but since MC totally think it was wonho and not ko-san, yeah that's man...