If wife disagrees and the husband still marries a second wife, that is extremely hurtful to the first wife. If God says we should treat them kindly, is such an treatment kind? Secondly, such an act can cause immense negative impacts on the first wife's emotions, and although it does not hurt them physically, it does emotionally which at times can even be worse. When God says we should not retain woman for injury/HURT (2:231), does this act cause great hurt to her? As we can see such an action goes against 2 of the ayaat of God, which clearly shows the immoral nature of it. Hence the injustice done to the first wife.
The consent of the first wife is a must, in order for it to be morally right, hence the treatment of one's wives with justice. Otherwise it is just unfaithfulness.