Okay, so... I think Kosuke is a decent guy.
He's certainly not perfect in any way, and has made a lot of bad decisions, but that just makes him human.
After his wife died, he tried to do his best for his family the only way he knew: by throwing himself into his work to provide a decent living for himself and his daughter.
Unfortunately, that meant that he failed to help his daughter deal with her grief, and she became a shut-in.
When he connected with Kisaragi, he tried to get her to help his daughter, because he had then realised that just food on the table wasn't enough support.
Somewhere in all this, Kisaragi's loss of her father and difficult relation to her mother meant that she was more receptive to Kosuke's parental care than his own daughter. This is unfortunate, but understandable.
Even when Kisaragi very clearly wanted to take their relation further, Kosuke did not accept it even though he was tempted (remember, he lost his wife, and had no other source of physical comfort). I suspect that a lot of people might have chosen differently, even if they regretted it later.
He never did give in to this temptation, despite her turning up at his business trip and other situations. The only instance of mutual inappropriate physical contact was the kiss in the park, and that was after things had already started to fall apart.
He also tried to give Kisaragi emotional support in her relation to her mother, and in dealing with the loss of her father. I don't know if he did it very well, but I do think that this was instrumental to her eventually being able to face her mother properly.
Now, he feels rejected by his co-workers, because they believe that he simply took advantage of a highschool girl. He feels rejected by his daughter (because he pretty much is). Worst of all, he feels rejected by Kisaragi because she has not been in contact with him for months after he decided to make a commitment to her (I'm not saying that doing that was a great idea, at least not in the way it was done, but it happened).
He is essentially worse off now than when the story began, despite attempting to do the right thing. Not always succeeding, mind you, but trying to.
I feel bad for him, all things considered.