Clearly you haven't read the entire manga, nor between the lines.
well, he did cheat. Thing's got a little tough, he's not getting sex, so he brings someone else back to his apt. If he hadn't been notified that Karin was missing, we don't know how much further it would have gone, but we know his intention . And he did leave Karin sitting at his apt for hours while he ran off to his ex girlfriend when her bike was supposedly broke. Once he arrived and he knew she wasn't in danger and was pulling a fast one he still didn't go back to the apt. to Karin who was waiting for him. I liked him less and less throughout the story.
I really don't like Kiriya for a. Cheating on Karin and b. Running to that ex girlfriend of his and leaving Karin behind so easily.