I guess it just shows why polygamy is a bad idea.
Koharu may have married for the wrong reasons, but she DID fall in love with Ryunosuke, and she really tried to be supportive of everyone, but now that Yuzu has the baby, she's the odd man out. On top of that, it seems that she's having problems getting pregnant, (it's been at least 3 years now, and she's not pregnant), so she feels any chance of Ryunosuke giving her the same level of attention he gives Yuzu and Rin is gone. She feels that she's essentially the maid or nanny, and not the wife.