Adopted sons are never the same as the biological sons from the loin!
The moral of the story above is that Allah Almighty wanted to tell us that "adopted sons" are no sons! Even in Sam Shamoun's bible, they do not inherit. So them being called "sons" is really pointless. A biological son or daughter who biologically belongs to both the father and the mother of the family (made from their own sperm and egg) is an actual son or daughter! Anything else is nothing but man made. That is why Allah Almighty Said:
"Behold! Thou didst say to one who had received the grace of God and thy favour: "Retain thou (in wedlock) thy wife, and fear God." But thou didst hide in thy heart that which God was about to make manifest: thou didst fear the people, but it is more fitting that thou shouldst fear God. Then when Zaid had dissolved (his marriage) with her, with the necessary (formality), We joined her in marriage to thee: in order that (in future) there may be no difficulty to the Believers in (the matter of) marriage with the wives of their adopted sons, when the latter have dissolved with the necessary (formality) (their marriage) with them. And God's command must be fulfilled. (The Noble Quran, 33:37)"
"God has not made for any man two hearts in his (one) body: nor has He made your wives whom ye divorce by Zihar your mothers: nor has He made your adopted sons your sons. Such is (only) your (manner of) speech by your mouths. But God tells (you) the Truth, and He shows the (right) Way. (The Noble Quran, 33:4)"
It was part of the Arabs' culture back then to adopt sons and to give them their last names. Zaid himself was adopted by Prophet Muhammad, peace be upon him, and people used to address as Zaid bin Muhammad. Allah Almighty saw with His Divine Wisdom that this pagan practice must end. To eliminate any confusions over the Laws of inheritance and genealogy, Allah Almighty once and for all eliminated the man-made corruption of having the "adopted sons" carry other peoples' family names and genealogies.
And as we clearly saw in the section above, Allah Almighty Commands Chastity and Honors it. Covetless societies is what Islam strives on building! Having men compete over the same woman, or having one man compete with another man over his wife is something that Islam, through practicality and realistic solutions, put an end to. Chaste-looking women who are covered very conservative cloths do not arouse evil in the society.
Like I mentioned above, women by nature can very dangerous, just as men by nature can be extremely brainless and stupid when they're sexually too high. Allah Almighty prevented the woman from abusing her natural and magical beauty and attraction to men by making it compulsory upon her to cover up, so that she can never become satan's tool to destroy the Chastity of the Muslim society.
So with all of this in mind, it is quite apparent that the marriage of Prophet Muhammad, peace be upon him, with Zaynab was not something that Allah Almighty wanted to be an excuse for sin and lust. It was rather a complete elimination of the so-called "adopted sons" absurdity, and a clear cut evidence that an ex-wife of a former "adopted son" is not a former "daughter in law"!
The Christian missionaires haven taken great delight out of the stories they find in Tabari's work, about Zaynab Bint Jahsh. Here is an example:
"The Messenger of God came to the house of Zayd b. Harithah. (Zayd was always called Zayd b. Muhammad.) Perhaps the Messenger of God missed him at that moment, so as to ask, "Where is Zayd?" He came to his residence to look for him but did not find him. Zaynab bt. Jash, Zayd's wife, rose to meet him. Because she was dressed only in a shift, the Messenger of God turned away from her. She said: "He is not here, Messenger of God. Come in, you who are as dear to me as my father and mother!" The Messenger of God refused to enter. Zaynab had dressed in haste when she was told "the Messenger of God is at the door." She jumped up in haste and excited the admiration of the Messenger of God, so that he turned away murmuring something that could scarcely be understood. However, he did say overtly: "Glory be to God the Almighty! Glory be to God, who causes the hearts to turn!"
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.
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!