It is best for a man to have only one wife, but Islam allows polygamy (or, more accurately polygyny) when social circumstances make it necessary. Islam placed important legal restrictions on polygyny, limiting it to one man with four wives, and also placed moral restrictions, encouraging men to take another wife only if they are capable of fulfilling their duties. A Muslim man who takes another wife should do so to fulfill his social responsibilities, such as caring for a widowed woman, and not as a means to increase his pleasure.
وَإِنْ خِفْتُمْ أَلَّا تُقْسِطُوا فِي الْيَتَامَىٰ فَانكِحُوا مَا طَابَ لَكُم مِّنَ النِّسَاءِ مَثْنَىٰ وَثُلَاثَ وَرُبَاعَ فَإِنْ خِفْتُمْ أَلَّا تَعْدِلُوا فَوَاحِدَةً أَوْ مَا مَلَكَتْ أَيْمَانُكُمْ
If you fear that you will not deal justly with the orphan girls, then marry those that please you of other women, two or three or four. But if you fear that you will not be just, then marry only one or those your right hand possesses…
Surah An-Nisa 4:3
Muhammad Asad comments on this verse:
As regards the permission to marry more than one wife (up to the maximum of four), it is so restricted by the condition, “If you have reason to fear that you might not be able to treat them with equal fairness, then marry only one,” as to make such plural marriages possible only in quite exceptional cases and under exceptional circumstances.
Source: Message of the Quran, verse 4:3
Therefore, polygyny is placed under an important moral restriction that the husband must treat all his wives equally, which should discourage a man from taking another wife simply to increase his pleasure.
وَلَن تَسْتَطِيعُوا أَن تَعْدِلُوا بَيْنَ النِّسَاءِ وَلَوْ حَرَصْتُمْ فَلَا تَمِيلُوا كُلَّ الْمَيْلِ فَتَذَرُوهَا كَالْمُعَلَّقَةِ
You will never be able to do perfect justice between wives even if it is your ardent desire, so do not incline too much to one of them so as to leave the other hanging…
Surah An-Nisa 4:129
Muhammad Asad comments on this verse:
This refers to cases where a man has more than one wife – a permission which is conditional upon his determination and ability to “treat them with equal fairness,” as laid down in verse 3 of this surah… In view of the fact that a man’s behavior towards another person is, in the long run, almost inevitably influenced by what he feels about that person, the above passage – read in conjunction with verse 3, and especially its concluding sentence – imposes a moral restriction on plural marriages.
Source: Message of the Quran, verse 4:129
The ideal of one wife can be seen in the example of the Prophet. He was married to his wife Khadijah for twenty-five years and he only remarried after she passed away. His plural marriages later in his life were due to his unique mission to be an example for the Muslim community, by taking care of widows, reconciling families through marriage, freeing slaves, and educating his wives in the teachings of Islam.
It should be noted that the Bible allows polygyny and places no restrictions upon it. For example, one verse reads:
Solomon he had seven hundred wives, princesses, and three hundred concubines…
Source: 1 Kings 11:3
Jews and Christians have no basis to criticize Islam for allowing polygyny, as their scriptures do not forbid it nor place any legal or moral restrictions upon it.
In fact, most human societies practice some form of polygynous marriages. According to the Ethnographic Atlas Codebook, of the 1,231 societies under study, 186 were monogamous, 453 had occasional polygyny, 588 had more frequent polygyny, and 4 had polyandry. Therefore, the view that polygyny is immoral is based upon a particular cultural norm that is not a universally recognized majority.
In conclusion, Islam allows polygyny but with important legal and moral restrictions. It is best for a man to have one wife so as to avoid jealousy and unfairness between co-wives, but it is permissible for a man to have up to four wives if he is capable of treating them fairly and his marriages will serve the interests of the common good.