She didn't cheat on him. They hadn't agreed to an exclusive relationship. They are both still in peachpuff lol (well, Noah is forced, but still, he hasn't asked her to stop that). Also, Noah was with pink haired boy when she was with tattoo boy (he was suffering from the drugs if I remember correctly, but he chose to have someone else relieve him instead of dealing with it himself). He didn't feel he owed fidelity in that moment and they didn't even talk about it because they hadn't established anything. Btw, I'm not trying to defend her, just setting facts straight. Too much unjustified hate towards her. Only in the last chapters they kind of started acting like exclusives.
she had sex with someone after they establish a exclusive relationship, twice! so yeas she did cheat on him, read it again if you don't remember. Noah forgive her for cheating of course cause that who he is, it dosn't make it better though.
he didn't have sex with the pink hair boy, the pink hair was the one who was jealous cause his guy was cheating on him with Noah girl soo....the pink hair guy wanted revenge and make it look like they doing something.
and for the fact, she did have sex with the other guy while dating exclusive with Noah. if it was up to Noah, he would have ignore it unlike the pink hair dude, that was so super XD

I kind of don't want to see Noah and Dohye =_= I'm already invested in noah and seyeon (not sure i remembered her name correctly though lol), so seeing the story of his first love throws me off (this always happens to me). I love yaoi, but I was already fixated on Noah and the girl. I hope the author develops more the bond between Noah and the girl in the future so their relationship doesn't feel like less than the one Noah and Dohye had.