Al-jawab billahi at-taufeeq (the answer with Allah's guidance)
Allah has created men and women as equal, but with different capabilities and different responsibilities. Men and women are equal in Islam, but not identical.
The reasons why Islam permits a man to marry more than one wife:
1) Taqwa (a God fearing attitude). 2) Preservation of vigour. 3) Even in case of disagreement divorce is avoided. 4) A wife may be effete- incapable of producing offspring. 5) In some countries, or families, there is a surplus of female children. 6) A woman does not generally remain fertile after fifty years of age while a man is fertile even at ninety years of age. 7) Adultery is very common in those countries where it is not allowed to marry more than one wife.
In Islam, Man is not allowed to marry more than four wives.
Allah Ta'ala says:
"then marry (other) women of your choice, two or three, or four but if you fear that you shall not be able to deal justly (with them).." (4:3)
The point is that it was necessary to set a limit otherwise men would tresspass the moderate limit and have innumerable wives. That would be harmful to themselves and the women. The problem is solved with four wives so it is not allowed to marry more than that.
(See:The wisdom Behind the commands of Islam Pg. 212-220)
The reasons why Islam prohibits a woman from having more than one husband:
1) Man is more polygamous by nature as compared to a woman.
2) Biologically, it is easier for a man to perform his duties as a husband despite having several wives. A woman, in a similar position, having several husbands, will not find it possible to perform her duties as a wife.
3) A woman who has more than one husband will have several sexual partners at the same time and has a high chance of acquiring venereal or sexually transmitted diseases which can also be transmitted back to her husband even if all of them have no extra-marital sex. This is less likely in a man having more than one sexual partner, and none of them having extra-marital sex. (Most common questions asked by non-muslims: By Dr. Zakir Naik)
