Before Islam, polygamy was considered normal and lacked any rules or limitations. As one of man’s essential needs, Islam has limited it, giving it specific conditions and terms. Islamic laws are all based on true needs and the benefit of society as a whole. The following needs are undeniable in society where people live in:
1- Men are a lot more prone to death than women are, that being a result of wars and other incidents that take place for them.
2- Men’s sexual urge lasts longer than women’s.
3- During their menstrual period and some of the time that they are pregnant, women aren’t allowed to have any intercourse, while men never have any prohibition in their lives whatsoever.
4- There are always women who for whatever reasons have lost their husbands, and in the case of the prohibition of polygamy, they usually have to remain widows for the rest of their lives.
These factors cause an imbalance between men and women, and in order for society to stay healthy, there are three things that can be done:
a) Men must be satisfied with only one wife, and widows need to stay widows for the rest of their lives and suppress all of their inner needs and feelings.
b) Men can only have one lawful wife, but can also have illegitimate sexual relationships with other women who have lost their husbands.
c) Men who have the ability to manage more than one wife, without any financial, physical or moral problems coming up, and can be just towards all of their wives and children, are permitted to marry more than one wife.
Out of the three, Islam has chosen the third. Also, there are many reasons behind why Islam doesn’t allow polygamy for women, such as: 1- Such a matter is in complete contrast with the nature of women. 2- The health of the future generation will be endangered. 3- The real parents of many children will be mixed up and unknown, and many other reasons.
After examining the condition of different environments before the advent of Islam, it can be concluded that polygamy was a normal issue at that time, with no limitations whatsoever, and it wasn’t Islam who suggested it for the first time. What Islam has done is count it as one of the necessities of social life and has limited it to many hard conditions and terms.
Islamic laws are all based on the true needs of man. If we take into consideration the benefit of society as a whole and put aside our personal feelings, the reasoning behind polygamy in Islam will become clear to us. No one can deny the fact that men are more prone to death than woman since they are the ones who are killed in wars and other incidents.
It also can't be denied that men’s sexual drive lasts longer than women, because women usually lose it at a certain age of their life.
In addition to the fact that women can't have intercourse during their menstrual period and for a part of their pregnancy, while men never have such limitations.
Moreover, there are women who for whatever reason have lost their husbands, and in the case of polygamy not being permissible, they usually have to remain widows for life.
Considering all of these factors (which cause a great imbalance between men and women), there are only three things that can be done regarding this problem:
1- Men must be satisfied with only one spouse, and widows need to stay like that for the rest of their lives and suppress all of their natural and inner needs and feelings.
2- Men can only bear one lawful wife, but they are free to have illegitimate sexual relationships with any widows they desire.
3- Those that have the ability to manage more than one wife without any physical, moral or financial problems coming up, and can observe justice towards all of them, be permitted to choose more than one wife.
The first way ends in numerous social problems and pressure on those women who have lost their husbands and in other words, is battle against their feelings and inner needs. But this is a battle that will never end in victory. Moreover, as is manifest to all, such a solution is inhumane.
In cases of its necessity, polygamy shouldn’t only be looked at from the first wife’s perspective, but should also be looked at from the second’s view and all of the different social advantages of such a matter need to be taken into consideration. Those who speak of the problems accompanying polygamy, are actually solely looking at one of the dimensions of a three-dimensional figure. If anyone is to reach a conclusion on this issue, he/she has to see the issue from three perspectives; the man’s, the first wife’s and the second wife’s perspective.
If the second way is chosen, we have to fully acknowledge fornication and sexual misconduct. In addition to the fact that this way only results in the fulfillment of sexual desires through widows, without them being supported financially and without them having any future, in addition to their dignity being trampled. No wise person would ever choose this way to solve the problem.
Therefore, the third method is the only one that remains which is both an answer to their natural and sexual needs, lacking any of the negative things that the other two methods entail.
This is the main reason behind why feelings and emotions have decreased in the West, resulting in showing feelings and care to animals and pets. It isn’t because in the West, they are necessarily loving and caring people. How can they be loving and caring, given the fact that they have killed thousands if not millions of innocent people in order to plunder their wealth, using the most brutal methods?!
As for why polygamy isn’t permissible for women, one must answer:
1- Such a matter is in opposition with the woman’s nature, because according to psychology, there is a considerable difference between their mental structures, tendencies and feelings, and that of men’s. Psychologists believe that women have a tendency to monogamy and naturally like to stay away from variety when it comes to husbands. They desire to have the emotional and practical support of one person. In fact, being after variety in this subject is counted as a disorder for them. On the contrary, men have a tendency towards variety in this issue, and as was said before, Islamic laws are all based on true needs and in accordance with the characteristics and the true essence of mankind.
2- The health of the future generation will be endangered.
3- The recognition of family trees and distinguishing them from each other will become impossible. No one will be able to tell which child belongs to which father. One of the things this necessitates, is the loss of familial emotions and the tendency to produce future generations. That is because naturally, everyone likes his/her own child and is only willing to invest on and strive for him/her. On the other hand, no one is ready to these things for a child that they aren’t sure belongs to them or not, and basically don’t know who he/she belongs to. Clearly, there usually isn’t any care and love to a child with such a status.
These are all some of the reasons behind why Islam has only permitted polygamy for men, not women.
 See: Makarem Shirazi, Tafsir Nemouneh, vol.3, pp.250-256; Morteza Motahhari, Majmou’eh Asar, vol.19, pp.357-361; Tafsir Al-Mizan, vol.4, pg.319.
 Morteza Motahhari, Majmou’eh Asar, vol.19, pg.302.
 For further reading, see: Morteza Motahhari, Nezame Huquqe Zan dar Islam; Abdollah Javadi Amoli, Zan dar Ayneye Jalal va Jamal; Seyyid Muhammad Hosein Tabatabai, Tafsir al-Mizan, vol.4.