Chika's character is realistic.
It might not seem to you, but I think it could be because of where you are from... maybe?
Because where I am from you can see this everyday. As I've seen it in heterosexual relationship context, I can assure you it has to do with the machismo of my culture (belief in the right of men to control women). And the thought of men aren't capable of controling their sexual drive, but that only shows how manly they are. They are manlier if they screw anything which wears a skirt (as the saying tells).
So cheating is thought as bad, but there is a lot of permission towards it because it is expected from men.
I think this is one of the reasons, but not the only one, because you can see dependeable relationships which go in the wrong way (hurting themselves a lot!) where the machismo isn't the only variable involved.
Chika's character not realistic?! where do you live in Jupiter?! Chika's character pretty much represents submissive partners. in hetero relationships, there are many women like Chika; specially in patriarchal societies like Japan. yup real Japan is a very conservative and patriarchal. in shoujo mangas you see many female characters very similar to Chika.
Chika's character can be realistic in any part of the world. They are usually people who have extremely low self-esteem and feel that they aren't good enough for anyone else, so they put up with the one who treats them badly. That's a typical and realistic situation just about anywhere.
But the feeling I get is that Chika can't feel hatred, as ugly as an emotion as it is. Even someone who has low self-esteem can realize that they're being treated terribly by some point in that sort of relationship. If they can't realize it themselves, then through someone else.
Yuuji is someone who ought to have learned his lesson when he started cheating, because he knows what he is doing is wrong and he only continues to do it because Chika doesn't do anything except to forgive . I reckon that Yuuji can only learn to change when he is forced to, in this case when Chika tried to leave for good.
In any case, Chika is someone who not exactly realistic, as most people would have let it go once or maybe even twice. But anything more would cause hatred, because many people would believe that they were only being played with.
Its a story about someon who is a pretty bad sinner and one very saintly person.