It is well known that the Prophet’s marriages were never to gratify lustful desires. Rather, if we take a look on every marriage he made, we will easily discern the wisdom behind it. It is no wonder to see the enemies of Islam touching such sensitive issues to cast a blemish on the personality of the Prophet (peace and blessings be upon him) for their goal is just to discredit Islam, as they find it unfathomable to see it as the world’s fastest growing religion. All their vicious attempts will come to nothing.
So first of all, we’d like to make it clear that none of the marriages was based on lust or uncontrollable passion, as some Orientalists would like us to believe. This fact is further clarified as follows:
“When people hear that the Prophet had many wives they conclude without much thought that the Prophet was a sensuous man. However, a quick historical review of his marriages proves otherwise.
When he was twenty-five years old he married for the first time his wife Khadijah, who was fifteen years older than he. She remained the only wife of the Prophet for the next twenty-five years until she died (may Allah be pleased with her).
Only after her death, did the Prophet (peace and blessings be upon him) marry other women. Now, it is obvious that if the Prophet was after physical pleasure he did not have to wait until he was more than fifty years old to start marrying more wives. He lived in a society in which it was quite acceptable to have many wives. But the Prophet remained devoted to his only wife for twenty-five years. When she died she was sixty-five years old.
His later marriages were for various reasons. Some marriages were with the view to help the women whose husbands had been killed while they were defending their faith. Others were with a view to cement relationships with devoted followers like Abu Bakr (may Allah be pleased with him). Yet others were to build bridges with various tribes who were otherwise at war with the Muslims. When the Prophet became their relative through marriage, their hostilities calmed down, and much bloodshed was averted.
Recent non-Muslim writers who had the opportunity to study the life of the Prophet first-hand reach a similar conclusion about his plural marriages.
John L. Esposito, Professor of Religion and Director of the Center for International Studies at the College of the Holy Cross, says that most of these marriages had “political and social motives” (Islam: The Straight Path, Oxford University Press, 1988, p. 19). This he explained as follows: “As was customary for Arab chiefs, many were political marriages to cement alliances. Others were marriages to the widows of his companions who had fallen in combat and were in need of protection.” (John L. Esposito, Islam: The Straight Path, pp. 19-20) Esposito reminds us of the following historical fact: “Though less common, polygyny was also permitted in biblical and even in postbiblical Judaism. From Araham, David, and Solomon down to the reformation period, polygyny was practiced.” (p. 19)
Another non-Muslim Caesar E. Farah writes as follows: “In the prime of his youth and adult years Muhammad remained thoroughly devoted to Khadijah and would have none other for consort. This was an age that looked upon plural marriages with favor and in a society that in pre-Biblical and post-Biblical days considered polygamy an essential feature of social existence. David had six wives and numerous concubines (2 Samuel 5:13; 1 Chronicles 3:1-9, 14:3) and Solomon was said to have had as many as 700 wives and 300 concubines (1 Kings 11:3). Solomon’s son Rehoboam had 18 wives and 60 concubines (2 Chronicles 11:21).
The New Testament contains no specific injunction against plural marriages. It was commonplace for the nobility among the Christians and Jews to contract plural marriages. Luther spoke of it with toleration.” (Caesar E. Farah, Islam: Beliefs and Observances, 4th edition, Barron’s, U.S. 1987, p. 69) Caesar Farah then concluded that Muhammad’s plural marriages were due “partly to political reasons and partly to his concern for the wives of his companions who had fallen in battle defending the nascent Islamic community.” (p. 69)”
The above quotation is excerpted, with slight modifications, from:
Now, to focus on the case in question, we would like to cite for you the following:
“The reason for this marriage must be understood in the light of some important details: Islam inherited the inhuman institution of slavery. There were scores of slave men and women in every house. Instantly freeing them, it is clear, would have resulted in a lot of social and economic problems. Islam, therefore, adopted a gradual methodology to do away with slavery. It undertook various measures in this regard.
However, freeing these slaves was not the only problem, which was to be tackled. An even more important problem was to blend and graft them within the normal social structure of the society once they had been set free. Keeping in view the great sense of superiority the Arabs had over slaves, this was an extremely uphill task.
Consequently, the Prophet (peace and blessings be upon him) in order to make them acceptable as normal members of a society took a very radical step. He persuaded his cousin sister Zaynab bint Jahsh to marry Zayd ibn Harithah, a slave boy he had set free and brought up as a son. The marriage took place, but, unfortunately, it could not continue due to certain reasons (Zaynab was rather unhappy about the marriage, because of Zayd’s former slave status. Zayd was very uneasy about the marriage and asked the Prophet’s permission to divorce her). After this unfortunate dissolution of marriage, the only thing which could console Zaynab was if the Prophet, peace and blessings be upon him, married her.
Furthermore, it was necessary to reform a social custom concerning some erroneous concepts about an adopted son. According to this custom, the Arabs regarded the adopted sons and foster sons equally in all respects. This, of course, is against human nature and as such had to be abrogated. However, as a social custom, it was so deeply rooted in the Arab society that it could only be the Prophet’s personality, which could abolish it. Consequently, on the Almighty’s bidding, the Prophet married her to sympathize with her and to reform this custom.”

So, in the light of the above, it’s clear that the Prophet’s marriage to Zaynab was for legislative reasons. A Divine order was revealed to him for this purpose, and it has to be borne in mind that the Prophet was very reluctant to do so, because of what people might say about his marrying his former “daughter-in-law”. But Allah, the Almighty, wanted to demonstrate in practice the absolute invalidity of adoption in the most practical manner.
More elaboration on this issue is made by the late Dr. Hammudah `Abdul-`Ati in his book Islam in Focus. He states:

“Before Islam, the Arabs did not allow divorcees to remarry. Zayd (may Allah be pleased with him) was adopted by the Prophet (peace and blessings be upon him) and used to be called his
son, in line with the custom among the Arabs before Islam. But Islam abrogated this custom and disapproved its practice.
Prophet Muhammad (peace and blessings be upon him) was the first man to express this disapproval in a practical way. So he married the divorcee of his “adopted” son to show that adoption does not really make the adopted child a real son of the adopting father and also to show that divorcees have a right to remarry. Incidentally, this very Zaynab was Muhammad’s cousin, and had been offered to him for marriage before she married Zayd.

The story of this Zaynab has been associated in some minds with ridiculous fabrications as regards the moral integrity of Muhammad (peace and blessings be upon him). These vicious fabrications are not even worth considering here.

Referring to the verses tackling the issue, Allah Almighty says: “And it becometh not a believing man or a believing woman, when Allah and His messenger have decided and affair (for them), that they should (after that) claim any say in their affair; and whoso is rebellious to Allah and His Messenger, be verily goeth astray in error manifest. And when thou saidst unto him on whom Allah hath conferred favor and thou hast conferred favor: Keep thy wife to thyself, and fear Allah. And thou didst hide in thy mind that which Allah was to bring to light, and thou didst fear mankind whereas Allah had a better right that thou shouldst fear Him. So when Zayd had performed the necessary formality (of divorce) from her, We gave her unto thee in marriage, so that (henceforth) there may be no sin for believers in respect of wives of their adopted sons, when the latter have performed the necessary formality (of release) from them. The commandment of Allah must be fulfilled. There is no reproach for the Prophet in that which Allah maketh his due. That was Allah’s way with those who passed away of old and the commandment of Allah is certain destiny. Who delivered the messages of Allah and feared Him, and feared none save Allah. Allah keepeth good account. Muhammad is not the father of any man among you, but he is the Messenger of Allah and the Seal of the Prophets; and Allah is Aware of all things.” (Al-Ahzab: 36-40)

These are the circumstances accompanying the Prophet’s marriage to Zaynab bint Jahsh. For the Muslims, there is no doubt whatsoever that Muhammad (peace and blessings be upon him) had the highest standards of morality and was the perfect model for man under all circumstances. To non-Muslims we appeal for a serious discussion of the matter. They, then, may be able to reach sound conclusions.”