In a nutshell:
The usual case of marriage in Islam is monogamous however polygyny is permitted. Islamic civilisations are based on large familial units, rather than individuals as is often seen in the West. Kinship ties and relations are intended to provide the functions we often see secular governments appropriating. As such, the larger the family in such civilisations, the more effective it is. The role of polygamy then facilitates larger families whilst polyandry would do the reverse by limiting reproduction. For such reasons men are permitted to marry more than one wife.
Polygyny in Islam is permitted. Polygyny is when a married man marries more than one unmarried women. Allah says,
وَإِنْ خِفْتُمْ أَلَّا تُقْسِطُوا فِي الْيَتَامَى فَانْكِحُوا مَا طَابَ لَكُمْ مِنَ النِّسَاءِ مَثْنَى وَثُلَاثَ وَرُبَاعَ فَإِنْ خِفْتُمْ أَلَّا تَعْدِلُوا فَوَاحِدَةً
"And if you fear that you will not be only in dealing with the orphans, then marry of other women as may be agreeable to you, two, or three, or four; and if you fear you will not be able to do justice, then marry only one and (the captives) you possess." (Qur'an al-Nisa' 4:3)
The husband is expected to be just and fair towards each wife. Perfect justice is too difficult to accomplish to is not expected (Qur'an An-Nisa' 4: 129).
The wife however can stipulate in the marriage contract that she does not want him to practice polygamy.
Islamic civilisations are usually based on large familial units, tribes or extended families, rather than individuals as is often seen in the West. Kinship ties and relations are intended to provide the functions we often see secular governments appropriating. As such, the larger the family in such civilisations, the more effective it is, increasingly self-sufficient and autonomous.
The role of polygyny then facilitates larger families whilst polyandry would do the reverse by limiting reproduction. For such reasons men usually marry more than one wife.
The Prophet (saw) said:
تَنَاكَحُوُا تَنَاسَلُوُا فَإِنِي مُبَاهٍ بِكُمُ الأُمَمَ يَوْمَ القِيَامَة
"Marry and beget children for I'll be proud of you before the nations on the Day of Resurrection." (Al-Bayhaqi)
Conclusion
Whilst monogamy is the usual form of marriage in Islam, polygyny is permitted in Islam, facilitating large, self-sufficient and autonomous families and tribes.
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