When Zayd came home, his wife told him that the Messenger of God had come to his house. Zayd said, "Why didn't you ask him to come in?" He replied, "I asked him, but he refused." "Did you hear him say anything?" he asked. She replied, "As he turned away, I heard him say: ‘Glory be to God the Almighty! Glory be to God, who causes hearts to turn!’"
So Zayd left, and having come to the Messenger of God, he said: "Messenger of God, I have heard that you came to my house. Why didn't you go in, you who are as dear to me as my father and mother? Messenger of God, perhaps Zaynab has excited your admiration, and so I will separate myself from her." Zayd could find no possible way to [approach] her after that day. He would come to the Messenger of God and tell him so, but the Messenger of God would say to him, "Keep your wife." Zayd separated from her and left her, and she became free.
While the Messenger of God was talking with 'A'isha, a fainting overcame him. When he was released from it, he smiled and said, "Who will go to Zaynab to tell her the good news, saying that God has married her to me?" Then the Messenger of God recited: "And when you said unto him on whom God has conferred favor and you have conferred favor, ‘Keep your wife to yourself .’"- and the entire passage.
According to 'A'isha, who said: "I became very uneasy because of what we heard about her beauty and another thing, the greatest and loftiest of matters - what God had done for her by giving her in marriage. I said she would boast of it over us." (The History of Al-Tabari: The Victory of Islam, translated by Michael Fishbein [State University of New York Press, Albany, 1997], Volume VIII, pp. 2-3; bold emphasis ours)
(Taken from : http://www.answering-christianity.com/lesbian_rebuttal.htm#zaynab)
In reply to that story, Abdul Hameed Siddique states in his book The Life of Muhammad PBUH:
" So far as the fanciful stories and calumnies of the Orientalists are concerned, we can only say that these are so absurd that any one having even a grain of sense in him would unhesitatingly reject them as mere fabrications. William Muir and so may others like him state that the Prophet, having seen Zaynab by change through a half-open door, was fascinated by her beauty, and that Zayd having come to know of the leanings of his master, divorced her and then she was marred ti Muhammad. There is absolutely no truth in these stories which have been fabricated in this connection."
(Source: The Life of Muhammad, by Abdul Hameed Siddique, Islamic Publications LTD, p. 214, bold and underlined emphasis ours)
The answer given by Abdul Hameed Siddiqui, doesnt really tell us much, so now I will quote from Sirat-Un-Nabi by Allama Shibli Nu'Mani:
" Tabari has it that once the Prophet (peace and blessings of Allah be upon him) visited the house of Zaid. Zaid was not at home, and Zainab was dressing herself. The Prophet (peace and blessings of Allah be upon him) saw her doing that, and turned back saying : " Glory to Allah, Most High and glory to him who turns the hearts". Zaid came to know of it. He came to the Prophet (peace and blessings of Allah be upon him) and said, " I may divorce Zainab if you have come to like her."
I have quoted that dirty narration with a pricking of conscience. But to report a blasphemy is not to commit a blasphemy. This is the single report that forms the mainstay of authority for the Christain historians. But the poor fellows do not know what value this narration holds when critically viewed in the light of the principles set by the traditionalists. Tabari, the historian has taken this story from Waqidi, the well-known liar and fabricator. He coined such fictions to provide some sort of sanction for the licentiousness of the 'Abbasid caliphs."
(Source: Sirat-Un-Nabi, by Allama Shibli Nu'Mani, rendered into English by M. Tayyib Bahksh Budayuni, Kazi Publications Lahore, Vol. II, p. 128-129, bold and underlined emphasis ours)
Shabir Ally also once stated:
Q. Is it true that the prophet (pbuh) had fallen in love with Zainab due to her beauty?
A. If that were true it would not detract from the veracity of the prophet. Muslims admit that he was a human being. It is not unnatural for a man to fall in love. The fact that he is a prophet does not rob him of his natural human emotions. In fact it is true that he loved his wives.
However, it is not true that he fell in love with Zainab in the way that is claimed by some critics. They say that once the prophet visited Zaid, the husband of Zainab. Zaid was out at the time, and Zainab was combing her hair. The prophet was struck by her beauty and immediately left saying something to the effect that God changes the hearts of people. When Zaid learnt about this incident he offered the prophet that he would divorce Zainab in order that the prophet may marry her. Accordingly, he divorced her and the prophet married her.
Several things point to the lack of truth in this story. First, it is unlikely that the prophet (pbuh) was suddenly struck by Zainab's beauty. Zainab was his cousin. He had known her since childhood. Why would she suddenly appear striking after she was already married to another?
Second, the prophet had arranged for her to get married to Zaid. If there was to be an attraction why did the prophet (pbuh) not encourage her to marry none but himself?
Third, the fact of the matter was that Zaid's marriage proved to be an unhappy one. Zaid was a former slave and as such was held in low esteem in the eyes of Zainab. He mentioned to the prophet that he intended to divorce his wife. But the prophet advised him to keep his wife and avoid divorce.
In the meantime, Zaid intended to divorce his wife, Allah intended to marry her to the prophet. Eventually Zaid could maintain his marriage no longer. He divorced Zainab and Allah declared in his Glorious Book that he has wedded her to the prophet after the proper waiting period was over.
This marriage served more than one purpose. First, the prophet was responsible for arranging Zainab's marriage to Zaid. In a sense, then, he was also indirectly responsible for the unhappiness she felt in her marriage. Her marriage to the prophet now provided her the honour she felt she deserved, and exonerated the prophet.
Second, Zaid had been adopted as the prophet's son. Eventually, however, the Qur'an prohibited the practice of changing the parental identity of adopted persons. Zaid, then, was to no longer be called "son of Muhammad" but rather "a close friend." The prophet's marriage to the divorced wife of Zaid was a practical demonstration that the adopted relationship was not equal to a real blood-relationship. A man cannot marry the divorced wife of his real son but he can marry the divorced wife of his adopted son.
The abolishment of the age-old practice was a positive improvement for the adopted persons. People outside of Islam still continue this practice for their own benefit. They adopt children and rob them of their real identify, making them believe they are real children of the household in which they grow up. When such children realize the truth they suffer much disappointment and grief. The adoptive process continues for the selfish gain of the adoptive parents.
But is it not true that children sometimes need adoptive parents? Yes. But they also need to preserve their real identify. This is what Islam ensures. It is the responsibility of the entire community to help children in need. They should be taken in and nurtured but not confused with one's own children.
The prophet's marriage to Zainab was a bold measure to forever engrave in the minds of his followers that as much as people would resist change, some changes are worth the effort. Adoptive children should no longer be robbed of their real identities.
(Source: http://www.islaminfo.com/new/detail.asp?ID=28)
And lastly, Sheikh Hammudah Abdallati stated:
6. Some of the Prophet's marriages were for legislative reasons and to abolish certain corrupt traditions. Such was his marriage to Zaynab, divorcee of the freed slave Zayd. Before Islam, the Arabs did not allow divorcees to remarry. Zayd was adopted by the Prophet (peace and blessings be upon him) and called his son as was the custom among the Arabs before Islam. But Islam abrogated this custom and disapproved of 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 marriage is lawful for divorcees. Incidentally, this very Zaynab was Muhammad's cousin, and had been offered to him in marriage before she married Zayd. He refused her then, but after she was divorced he accepted her for the two legislative purposes: the lawful marriage of divorcees and the real status of adopted children. The story of this Zaynab has been associated in some minds with ridiculous fabrications regarding the moral integrity of Muhammad. These vicious fabrications are not even worth considering here (see Qur'an, 33: 36, 37, 40).
(Source: Islam in Focus, p.177-179 by Hammudah Abdallati, bold and underlined emphasis ours)
So we can safely conclude, that the narration given by the missionaries, is wrong, and can NOT stand up to the light of the principles set by the traditionalists.
And Allah SWT Knows Best!