Polygamy is necessary in certain instances, like when the number of women are much larger then the number of men due to battles and such. But lets not forget the clear guideline that is in the Quran with regard to polygamy.
Surah Nisa, verse 3
"If ye fear that ye shall not be able to deal justly with the orphans, marry women of your choice, two, or three, or four; but if ye fear that ye shall not be able to deal justly (with them), then only one, or that which your right hands possess. That will be more suitable, to prevent you from doing injustice."
It is quite clear that the four women should be treated equally in the Quran, and if you fear injustice then marry only one. The Quran actually does prohibit polygamy for the men who cannot treat his wives justly.
On a side note, I think Muslims are contradicting themselves here. How so? Muslims often tell us that Muhammad put an end to the rampant seventh-century practice of female infanticide. According to Muslim apologists, there was an epidemic of parents murdering female babies. Well now, if people were regularly killing their female babies, there would be fewer females than males, wouldn't there? But then these same Muslim apologists, when they defend polygamy, assure us that there were so many women in Arabia, Muslims needed to marry two, or three, or four (or far more, in Muhammad's case) to take care of all the women (and this isn't even counting all their sex slaves). Quite an inconsistency, I think. But there's a more important issue.
Samatar declares, following the Qur'an, that a Muslim must treat his wives equally. Hence, if a Muslim doesn't treat his wives equally, he is sinning. But Muhammad himself didn't treat his wives equally.
Consider this Hadith, which shows Muhammad's wives pleading with him to treat them the way he treats Aisha (who was obviously getting special treatment):
Sunan An-Nasa'i 3396—Aishah said: "The wives of the Prophet sent Fatimah, the daughter of the Messenger of Allah, to the Messenger of Allah. She asked permission to enter when he was lying with me under my cover. He gave her permission to enter, and she said: 'O Messenger of Allah, your wives have sent me to you to ask you to be equitable with regard to the matter of the daughter of Abu Quhafah.' I (Aishah) kept quiet and the Messenger of Allah said to her: 'O my daughter! Do you not love the one whom I love?' She said: 'Yes.' He said: 'Then love this one.' Fatimah stood up when she heard this and left the Messenger of Allah, and went back to the wives of the Prophet. She told them what she had said, and what he had said to her. They said to her: 'We do not think that you have been of any avail to us. Go back to the Messenger of Allah and say to him: Your wives are urging you to be equitable with regard to the matter of the daughter of Abu Quhafah.' Fatimah said: 'No, by Allah; I will never speak to him about her again.'" Aishah said: "So the wives of the Prophet sent Zainab bint Jahsh to the Messenger of Allah; she was one who was somewhat equal to me in rank in the eyes of the Messenger of Allah. And I have never seen a woman who was better in religious commitment than Zainab, more fearing of Allah, more honest in speech, more dutiful in upholding the ties of kinship, more generous in giving charity, and devoted in giving herself in acts of charity, by means of which she sought to draw closer to Allah. But she was quick-tempered; however, she was also quick to calm down. She asked permission to enter upon the Messenger of Allah when he was with Aishah under her cover, in the same situation as when Fatimah had entered. The Messenger of Allah gave her permission to enter and she said: 'O Messenger of Allah, your wives have sent me to ask you to be equitable with regard to the daughter of Abu Quhafah.' Then she verbally abused me at length, and I was watching the Messenger of Allah to see if he would allow me to respond. Zainab went on until I realized that the Messenger of Allah would not disapprove if I responded. Then I spoke back to her in such a way, until I silenced her. Then the Messenger of Allah said: 'She is the daughter of Abu Bakr.'"
So Muhammad showed favoritism to Aisha above his other wives. But this makes him a horrible sinner, according to Samatar.
Muhammad's companion Umar even warned his daughter Hafsa not to expect Muhammad to treat her equally, for Muhammad loved Aisha more than her.
Sahih al-Bukhari 3:648— . . . Then ‘Umar went on relating the narration and said. "I and an Ansari neighbor of mine from Bani Umaiya bin Zaid who used to live in ‘Awali Al-Medina, used to visit the Prophet in turns. He used to go one day, and I another day. When I went I would bring him the news of what had happened that day regarding the instructions and orders and when he went, he used to do the same for me. We, the people of Quraish, used to have authority over women, but when we came to live with the Ansar, we noticed that the Ansari women had the upper hand over their men, so our women started acquiring the habits of the Ansari women. Once I shouted at my wife and she paid me back in my coin and I disliked that she should answer me back. She said, ‘Why do you take it ill that I retort upon you? By Allah, the wives of the Prophet retort upon him, and some of them may not speak with him for the whole day till night.’ What she said scared me and I said to her, ‘Whoever amongst them does so, will be a great loser.’ Then I dressed myself and went to Hafsa and asked her, ‘Does any of you keep Allah’s Apostle angry all the day long till night?’ She replied in the affirmative. I said, ‘She is a ruined losing person (and will never have success)! Doesn’t she fear that Allah may get angry for the anger of Allah's Apostle and thus she will be ruined? Don’t ask Allah’s Apostle too many things, and don't retort upon him in any case, and don't desert him. Demand from me whatever you like, and don’t be tempted to imitate your neighbor (i.e. ‘Aisha) in her behavior towards the Prophet, for she (i.e. Aisha) is more beautiful than you, and more beloved to Allah’s Apostle.
Muhammad's wife Sauda even had to surrender some of her marital privileges to Aisha in order to please Muhammad.
Sahih Muslim 3451—A’isha (Allah be pleased with her) reported: Never did I find any woman more loving to me than Sauda bint Zam'a. I wished I could be exactly like her who was passionate. As she became old, she had made over her day (which she had to spend) with Allah's Messenger (may peace be upon him) to ‘A’isha. She said: I have made over my day with you to ‘A’isha. So Allah's Messenger (may peace be upon him) allotted two days to ‘A’isha, her own day (when it was her turn) and that of Sauda.
Sahih al-Bukhari 3:766—Narrated Aisha: Whenever Allah’s Apostle wanted to go on a journey, he would draw lots as to which of his wives would accompany him. He would take her whose name came out. He used to fix for each of them a day and a night. But Sauda bint Zam’a gave up her (turn) day and night to ‘Aisha, the wife of the Prophet in order to seek the pleasure of Allah's Apostle (by that action).
So Muhammad ended up spending more time with Aisha than with any of his other wives, and he ended up spending no time at all with Sauda!
And yet Samatar tells us that Muslims must treat their wives equally. In doing so, he condemns his own prophet. Interestingly, the Qur'an declares that Muhammad is an excellent moral example for Muslims to follow. It seems, then, that Muslims are commanded to follow the example of a man who was a horrible sinner and couldn't even follow the simplest commands of the Qur'an.
***UPDATE*** Samatar can't help himself. In his most recent comment, he writes:
The Quran says to treat your wives equally. When I said equally, I meant in the sense of food, clothing, sex etc... I did not mean favouring one wife over another, Or loving one wife more than another.
So the Qur'an calls Muslims to treat wives equally in regards to sex? But Muhammad spent an additional night for sex with one of his wives (Aisha), and had no sex with a less favored wife (Sauda). So one wife got no sex, some wives got some sex, and Aisha got the most sex. Hence, even on Samatar's modified position, Muhammad still turns out to be a hypocrite and a sinner in rebellion against Allah's clear commands in the Qur'an.
Samatar also wrote:
With regard to Sahih al-Bukhari 3:766, it is quite clear that Sauda gave up her night with the prophet willingly, so there is no injustice their. It would only be injustice if the prophet (pbuh) spent the night with Aisha against Sauda's permission.
Samatar, I beg you, please start reading your sources. Sauda gave up her night with Muhammad because he was going to divorce her, leaving her as an old and starving widow. So she made the agreement in order to survive, and Muhammad agreed because it allowed him to spend more time with his child-bride. If Muhammad were a good moral example, he would have responded, "Sauda, what do you mean you're worried that I will divorce you? I will never divorce you, because I've been commanded to care for all my wives equally. So there's no need to give up your evening with me, for that would result in unequal treatment, which the Qur'an forbids." Instead, Muhammad's response was, "What? You're giving your sex night to Aisha? Great! Okay, you can continue receiving food and shelter, and I'll have double sex with Aisha! Win/win!" And the Qur'an even condones this behavior (4:128-130).
Samatar, do you have the slightest clue what a repulsive hypocrite Muhammad was? He was a thoroughly despicable excuse for a human being, and all you do is defend him. This is what Islam does to people. It forces them to defend the indefensible.