It is commonly understood that Islam allows a man to have up to four wives, and many defend this practice as beneficial to society, in that it is a thoughtful way to care for [surplus!] grown women who would otherwise have no home or protection, Indeed, that is what Muhammed taught specifically, that women should be grateful for the care of a man, even if she had to share him with other women. Her options were limited, and she might otherwise be homeless or forced into prostitution.
This Islamic site explains the responsibility:
Notice that the Qur'an permits but does not command a man to have four wives. Furthermore, the Qur'an stipulates that a man is responsible for the maintenance of his wife or wives. If a man has more than one wife, he has to provide separate living accommodation for each of his wives. Multiple marriages are a heavy responsibility on the male. It is not a pleasure trip as some people may assume. Some even imagine all kinds of sexual exploits involving a man and his wives altogether. However, such activity is not permissible in Islam. A man must divide his time equally among his wives. He may, for example, spend one night with each wife on a rotating schedule. If a man cannot maintain justice in the treatment of his wives, the Qur'an stipulates that he is to have no more than one wife.
Raymond Ibrahim looks deeper into the question of "why four?" Referencing an Arabic program that addressed the question to a Islamic experts:
One of these clerics responded by saying that, before Islam, there was no limit to how many wives an Arab man could have. But when Koran 4:3 appeared, telling Muslim men to “Marry such women as seem good to you, two and three and four,” a man came to Islam’s prophet Muhammad saying, I have 19 wives, what do I do now? Muhammad told him, keep four and divorce the rest.
The cleric then said that in another hadith, Muhammad said a woman should be married for one of four reasons: 1) her beauty, 2) her lineage, 3) her wealth, and 4) her piety.
However, argued the cleric, it is impossible to find all four traits in one woman. That is why a Muslim man, who deserves the best that womanhood can offer, is permitted to have four wives—ideally, one that represents each of the four aforementioned traits sought after in women.
There are many difficulties with this approach to personhood. The first is that a woman is pidgeon-holed into a single attribute, on which she must rely on for her acceptance to the husband. Imagine being chosen for your piety, and knowing that you are simply not beautiful in your husband's eyes. Or that you were chosen specifically for your beauty, which may fade. Imagine knowing that you were chosen -- not because of your own attributes -- but because your family lineage was a trophy to your husband, or being the fourth chosen specifically for her wealth, which would then be spent on the other three wives as well as yourself (the beautiful one, the pious one, and the noble one -- the qualities you evidently lacked).
Polygamy for baser reasons is almost easier to accept, knowing that you were simply chosen for yourself, but knowing that you had to share your husband. This explanation is more degrading, it would seem, and three of the wives -- who must be given equal time -- would know that they were not the beautiful one, which has a unique compelling force in love.
Jealousies and rivalries are inevitable when marriage is arranged this way. But the brick over each woman's head is this verse in the Qur'an:
Perhaps his Lord, if he divorced you [all], would substitute for him wives better than you - submitting [to Allah ], believing, devoutly obedient, repentant, worshipping, and traveling - [ones] previously married and virgins.
All the wives are replaceable. They remain in the home only as long as the husband is satisfied. This is no way to build a relationship, nor secure ground on which each woman and her children can find lasting comfort. If divorce is a man's prorogative, the woman is not a partner but a object, fulfilling a task. If a woman has one-quarter of a man's time and affection, it is not a marriage but a relgiously sanctioned brothel.
I understand that there are many satisfying marriages with deep attachments within the Umma. I would posit that these are a testament to natural law and the extraordinary bonds that are possible between men and women. They exist -- not because of Islam -- but despite it.