Interesting to see how divided the comments are.
Remember, Ruka is his girlfriend. Period. Chizuru is an escort (sorry, "rental girlfriend.") If anyone is the "other woman" or "homewrecker" in this, it is Chizuru.
Don't get me wrong, I think I'd pick Chizuru over Ruka as well. However, pursuing Chizuru while he has a girlfriend is not the girlfriend's fault. He's the one in the wrong, not her. She has every right to be upset that her boyfriend is messing around. MC is a spineless loser who is being totally unfair to his girlfriend. He needs to grow a pair and break up with her if he wants Chizuru and has no interest in Ruka.
(Side note, I dunno if I could handle Chizuru after all. I want a devoted woman and she's downright said that she loves her job as an escort, and loves dating different men 5 nights a week or whatever. It is probably intentional on the part of the author that we don't really see that stuff going on. Call me possessive but I'm not down with that from my significant other. Ruka quit and didn't look back. Other than when Chizuru is on the clock, Ruka's been a far better girlfriend.)