Polygamy means a system of marriage whereby one person has more than one spouse.
Polygamy can be of two types. One is polygyny where a man marries more than one woman, and the other is polyandry, where a woman marries more than one man. In Islam, limited polygyny (up till four wives) is permitted; whereas polyandry is completely prohibited.
Qur’an is the only religion on the face of the earth that allows men having up till four wives, Allah says in the Holy Qur'an:"Marry women of your choice, two, or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one."
Before the Qur’an was revealed, there was no upper limit for polygyny and many men had many wives, some even hundreds. Islam put an upper limit of four wives. Islam gives a man permission to marry two, three or four women, only on the condition that he deals justly with them.
So, Islam came to control having an infinite number of wives, like it was during the Pre-Islamic era.
In the same chapter Allah says in Surah Nisa verse 129, Allah says:"You will never be able to be fair and just among women (wives)...."
Hence, Islam does allow having more than one wife, yet it has set conditions for that, which is that the husband should be fair with all his wives, and treat them equally, which is very difficult.
Multiple marriages lay a heavy responsibility on men, which is why Allah allowed having more than one wife only if the husband treats his wives equally. If a man has more than one wife, he must treat them all in an equal manner, emotionally and financially. For example, he has to provide separate living accommodation for each of his wives.
Since it is very difficult to be just with all wives, in practice, most of the Muslim men do not have more than one wife.
Therefore polygyny is not a rule or an order but an exception. Many people are under the misconception that it is compulsory for a Muslim man to have more than one wife.
The reason for multiple wives in Islam was not to satisfy men’s desire and that's it, but for the welfare of the widows and the orphans of the wars. During war times, many women are unable to find husbands, and they might prefer to be a co-wife than no wife.
Also allowing multiple marriages is useful in case a man's wife is sterilized and can't give him the chance of being a father; here he has the right to marry another wife, and leaving the choice to her whether she wants to continue being his wife if he marries another woman or not
However men are prohibited from cheating on their wives, meaning a man can't marry another woman without the knowledge of his wife. He should tell her, for she might refuse such situation, and in this case it's totally her right if she asked for divorce.
In the pre-Islamic period, men used to have many wives.
One person had 11 wives and when he converted to Islam, he asked Prophet Mohammad (pbuh): "What should I do with my wives?"
Prophet Muhammad (PBUH) replied:"Divorce the all except four."
hear about Prophet Mohammad’s multiple wives, they might conclude without much thought that he was a sensuous man. But the truth is that Prophet Mohammad (PBUH) was married to only one woman, Khadijah. He remained devoted to her from age 24 to 50. Only after her death, did the Prophet marry other women. The reason behind the Prophet (PBUH) marrying more than one wife was to spread Islam, and help the Islamic message to reach out to the world.
It is obvious that if he was after physical pleasure, he didn’t have to wait until he was more than fifty years old to start marrying other wives. His later marriages were for various reasons, some marriages were with the view to help widows whose husbands had been killed for the sake of Islam, others were to cement relationships with devoted followers like Abu Bakr Al-Sadeeq, may Allah be pleased with him. Some of his marriages were to build strong relations with various tribes that were at war with Muslims. In brief, Prophet Mohammad’s marriages always had Islamic motives.
Some of the non-Muslims misunderstand polygyny and claim that it is one of the negative aspects of Islam, but in fact Islam sought to control and regulate the number of wives rather than give free license. And the permission given to men to have more than one wife is controlled with the conditions Allah has set before marrying another wife.