Answer
Assalamu Alaikum Dear Sister,
Thank you for your question and for contacting Ask About Islam.
When the fullness of revelation dawned upon the Arabian Peninsula in the 7th Century it was normal at that time for a man to take as many wives as he wanted and he could treat them any way he wished. This had been so for centuries.
Indeed, in the Bible (First Kings 11:1-3) we read that in the past King Solomon had seven hundred wives and three hundred concubines.
Islam came to bring a new way of living in which all people were shown respect. Women were not to be treated as possessions, but were to be given rights.
In marriage, for example, a woman could keep her own money and would not be required to contribute financially in any way to the marriage. It would be the duty of the husband to provide for her. Similarly, she could keep her own name, since she did not belong to anyone.
In the early years of Islam, when many of the Muslim men were being killed on the battlefield, many women were left without anyone to provide for them. In those days, if you were not married there was no social security to provide for you.
In such a case, Allah revealed that it was possible for a man to marry up to four wives, if he could care for each of them equally. They were not his possession, and in marrying them he was doing something more pious in providing for those without comfort and support.
If some Muslims today mistakenly believe that they can take additional wives for their pleasure alone, they are wrong.
In taking another wife, a Muslim man should look for one more pious. In dealing equally with wives, he should bear in mind the feelings of his other wives, not disregard them because it is what he wants to do.
The law allows him to take up to four wives, but his own Judgment on the Last Day will reflect the way he treated them.
Finally, we must ask what the reason would be for a woman to take a second husband, if she already has one to support and protect her. This does not make sense.
It is no excuse, though, for a man to behave in a way that is less than just and loving. The way he treats his wife/wives will be the way he is judged.
Please read:
Men and Women: Equality or Justice?
I hope this helps answer your question.
Salam and please keep in touch.