I don't think they black haired guy counted as cheating since he was drunk.
So maybe 'cheating by accident' since both were drunk > realizes he has feeling for person he "cheated" with.
The proper way of course sorted his feelings and relationship with his current girlfriend (one he was cheated from).
Now, the light haired guy..... NOW THAT was crime. And the fact that they were not both drunk. Black haired dude wasn't cheating. He was raped.
But from plot view, that was good twist.