Islam places great emphasis on a couple to keep each other happy, if a wife does not agree to you taking another wife it will upset her if you do & hence cause a rift in the marriage which is not endorsed by Islam, i am no expert in Islam, this is just common sense, i also believe that if (as some say) a man can marry again without the first wifes permission then this refers to an extreme case where the marriage would greatly benefit the second wife who may at the time be suffering a harsh life, like every teaching in the quran, this must also have a logical reason, & as in Islam women are not regarded as slaves or second class citizens then the husband has no right to upset his wife in this way as she does not have the same right, hence the only logical reason for allowing a man to marry another woman against the wish of the first wife in my opinion is that the other woman situation is so bad that the first wife would later realise that the second marriage was better than to leave the woman in he previous circumstances. This law was not created to allow men to take further brides against their wifes will for selfish reasons…thats called abuse which Islam does not allow.
The only men allowed to have more than one wife are the men so great, so respected & highly held by their wives that they truly believe in their hearts that they alone should not be the only women to have such a man in their life & so are genuinely happy to be able to share the life they have with other women.
Such was the case (i believe) with the prophet & the women in his life…he was a remarkable human being, very few can be like us…