There are three basic reasons for marriage in Islam:
The subject of polygamy generates more misconceptions about Islam than any other. Islam does allow polygamy, i.e., having more than one wife at the same time, but it does not encourage it. In fact, Islam is the only religion that limited this ancient and widespread practice. Previous teachings permitted unlimited and unrestricted polygamy. Islam limits the number of wives allowed to four, and also discourages the practice. Allah says:
“But if you feel you many not be able to deal justly between them, then marry only one.” (Holy Qur’an, 4:4).
If a man cannot be satisfied with one woman, it is better for society that he assumes full moral and financial responsibility of a second wife. Having a mistress and committing adultery is a grievous sin and causes the moral decline of society.
Polygamy becomes a necessity under certain circumstances, but can only be practiced under certain conditions and with restrictions. A man has been given permission to marry again if his wife is chronically ill and cannot fulfil the obligations of marriage; or if she is unable to have children; or under certain conditions of wartime, when marrying widows to look after orphans may be necessary to protect the morals of society. Also it must be emphasized that Ahmadiyya Muslim Community’s policy requires that Ahmadi citizens obey the law of the land in which they live as long as that law does not conflict with an express command from Allah.
Now, the next question is why permission has been given for up to four wives. Why not three or five? “Four” (cardinal number) was a sacred and complete number with the Hebrews, as well as with several other peoples. It occurs very frequently in the Old Testament and the New Testament. It indicates completeness. Number 4 is the number of stability, order and completion of justice. Four is a cardinal number in prior scriptures. Arba in Hebrew and Arabic means 4. In the Jewish scripture and saying of their sages the number four signifies completeness, wholeness, or fullness. Allah in His wisdom has determined four to be an adequate number to provide proof beyond doubt (four witnesses) or to provide adequate time. It represents comprehensiveness, wholeness and fullness or completion. Also there are four cardinal attributes of Allah : Rabb (Lord), Rehman (Gracious), Raheem (Merciful) and Malik (Master).
Women who are married to chronically ill, sterile or impotent husbands are allowed recourse to divorce, if they feel the situation is unbearable.
Marriage is between a man and a woman. A woman can have only one husband at a time. This is Divine intent and design. If man’s actions and man made laws are not congruent with Divine intent and design there are consequences. Islam forbids polyandry which is when a woman is married to more than one husband at the same time. One reason for this is that it is not physically possible to bear children of multiple husbands at the same time; one of the many reasons this has not been institutionalized throughout the ages. And also there will always be confusion and controversy about “whose the father of the child” leading to medico-legal issues. Hence polyandry does not serve the purpose of marriage and will create more emotional and practical problem than the restricted polygamy solves in certain circumstances.
Another issue with polyandry and polygamy being permitted at the same time in the name of equality is that if one woman can have multiple husbands, then the husband can also have multiple wives, and there is no limit on these extended relationships and the family unit cannot be easily defined.
 The Islamic Marriage System, Pathway to Paradise.
 Holy Quran, 1:2-4