Hi – regarding this question, I have always felt that the requirement for husbands to be fair to all his wives to be treated lightly in most muslim countries, with no sufficient legal provision to compel errant husbands or ex-husbands (for child support) and no enthusiasm to correct the injustice, nor is there social recognition for a wife to choose to refuse to be part of a shared marriage.
In my view, considering the contractual nature of marriage in Islam (as opposed to ‘sacramental’ marriage in Christianity), if it was understood at the time of the contract that the man is promising to provide for one family (i.e. the one with the bride), if he wishes to take a second wife, would this not be a modification of the original contract, and should not the other party retain the right to agree or disagree to the modification? In all other contracts where one party promises one thing, but later wants to add or remove a condition, does not that open the contract to be re-negotiated by the other party as well, without the other party being subjected to disapproval? If not, then marriage is not really a contract, if only one party has the right to unilaterally change the conditions of the contract without any say from the other.
The marriage contract is pretty much anything agreed upon by the parties involved. So, if the parties mentioned that there would not be a second marriage and then the husband proceeds with another marriage then there is a breach of contract. However, if no mention is made of a second, or more, marriages then no violation has been committed. It should be kept in mind that the marriage contract does not automatically postulate that there is a contractual obligation to only one wife. Rather, the very setup of the contract is to establish that the husband will commit to his duties and live up to his obligations to that wife. In other words, it neither limits him as to whom he takes care of nor does it require him to max out his resources. His primary concern is to fulfill his obligations to his household.
It should be kept in mind that the contract may consist of whatever agreement between the two parties is made. This may include, but is not limited to, stipulations regarding any other marriages. Of course, one cannot ask that a contract contain any unfair or unjust conditions. Keeping the foregoing in mind, it should understood that if there is no mention of multiple marriages the contract does not automatically modify the first by virtue of more responsibility that is lived up to without taking away rights from the first.
I hope this helps.
God knows best.