How did the Prophet (peace and blessings of Allah be upon him) marry Khadeejah?
Question

Many of my friends have asked me about having Affairs with there opposite sex.........Now I have told them this is Haram in Islam but they give me the reply by saying that Khadija (R.A.A.H) fell in love with out Prophet (S.A.S) And had married him. I have read a lot of books and even on the net i havent found any information about how they were married and all i knew was Khadija (R.A.A.H) SENT a message though a Slave girl who was the cousin of our Prophet (S.A.S). And Abu Talib (R.A.A.H) uncle of our Prophet (S.A.S) accepted the offer and wed them. Now i wanted to know that have they (Prophet (S.A.S) and Khadijah (R.A.A.H)) ever met prior to marraige?.
Praise be to Allah.The reports of the seerah (Prophet’s biography) indicate that Khadeejah bint Khuwaylid (may Allah be pleased with her) was a determined and intelligent woman, and she was also rich and had several kinds of trade. The men of her people were keen to marry her. She did not engage directly in trade herself, rather she used to employ men to work on her behalf. 
News reached Khadeejah of the honest and trustworthy Muhammad (peace and blessings of Allah be upon him); she heard a great deal about his honesty and trustworthiness, so she wanted to hire him to work for her and do trade on her behalf. She sent someone to him with an offer of work, and he (peace and blessings of Allah be upon him) agreed to that. 
The Prophet (peace and blessings of Allah be upon him) went out to do trade with the money of Khadeejah (may Allah be pleased with her), and there was with him a slave of Khadeejah’s whose name was Maysarah.  Maysarah saw the signs that happened to the Prophet (peace and blessings of Allah be upon him) whilst travelling, and that amazed him, so he started to tell his mistress Khadeejah about everything that he had seen. 
For example, when the Messenger (peace and blessings of Allah be upon him) came to the city of Busra in Syria, he stopped in the shade of a tree, and one of the monks said to Maysarah, “No one ever stops under the shade of this tree but a Prophet.” And Maysarah used to see two angels shading the Prophet (peace and blessings of Allah be upon him) when the heat of the sun grew too intense. 
The Prophet (peace and blessings of Allah be upon him) came back from that journey, and he had made far more money in trading for Khadeejah than anyone else ever had before. Khadeejah was impressed with the personality of the Prophet (peace and blessings of Allah be upon him) and she wanted to marry him. So she sent her friend Nafeesah bint Maniyyah to tell the Prophet (peace and blessings of Allah be upon him) about that. He (peace and blessings of Allah be upon him) agreed, and the marriage of Khadeejah (may Allah be pleased with her) was arranged by her father Khuwaylid, according to the most sound reports, as mentioned by the scholars of seerah. 
From the above it is clear that there was no inappropriate relationship between the Prophet (peace and blessings of Allah be upon him) and Khadeejah (nay Allah be pleased with her) before he married her. 
The morals of the Prophet (peace and blessings of Allah be upon him) were sound and he lived a virtuous life; Allah protected him from everything that may have detracted from his message or that was contrary to modesty and chivalry.
If such a thing has happened – but he (peace and blessings of Allah be upon him) is far above such a thing – the kuffaar of Quraysh would not have kept quiet about it. They would have regarded it as a great source of shame and they would have used it as a means to reject his religion and tarnish his reputation. But nothing of the sort happened, rather before his Prophethood they used to call him al-Saadiq (the honest one) and al-Ameen (the trustworthy one). They did not cast aspersions upon his chastity at all. 
You should also note that the Prophets (peace be upon them) – all the Prophets – are the most perfect and best of human beings. Allah does not choose anyone to convey His Message except those who are fit for it. Allah says (interpretation of the meaning): 
“Allah knows best with whom to place His Message” [al-An’aam 6:124]
Ibn Katheer (may Allah have mercy on him) said: 
This means, He knows best where to place His Message and who is most fit for that among His creation. This is like the verse (interpretation of the meaning): 
“And they say: ‘Why is not this Quran sent down to some great man of the two towns (Makkah and Taa’if)?’
 
Is it they who would portion out the Mercy of your Lord?” [al-Zukhruf 43:31]
What they meant was, If this Quran had been sent down to some great man or leading figure who was of high status in their eyes, “of the two towns” i.e., Makkah and al-Taa’if. That was because they – may Allah curse them – despised the Messenger (peace and blessings of Allah be upon him) out of hatred and envy, and stubbornness and pride, as Allah tells us (interpretation of the meaning): 
“And when those who disbelieved (in the Oneness of Allah) see you (O Muhammad), they take you not except for mockery (saying): “Is this the one who talks (badly) about your gods?” While they disbelieve at the mention of the Most Gracious (Allah)” [al-Anbiya’ 21:36]
And Allah says (interpretation of the meaning): 
“And when they see you (O Muhammad), they treat you only in mockery (saying): ‘Is this the one whom Allah has sent as a Messenger’” [al-Furqaan 25:41] 
“And indeed (many) Messengers before you were mocked at, but their scoffers were surrounded by the very thing that they used to mock at” [al-An’aam 6:10]
This happened even though they were aware of his virtue, nobility and lineage, and the purity of his house and upbringing.  Before the Revelation came to him, they used to refer to him amongst themselves as al-Ameen (the trustworthy one). The leader of the kuffaar, Abu Sufyaan, admitted that when Heraclius, the ruler of Rome, asked him. “What is his lineage amongst you?” He said, “He is of noble descent.” He asked,”Did you ever accuse him of lying before he said what he said?” He said, “No.” In this lengthy conversation the Roman ruler took the pure attributes of the Prophet (peace and blessings of Allah be upon him) as evidence that his Prophethood was real and that what he said was true. 
The scholars (may Allah have mercy on them) stated that the Prophets were protected from committing major sins and every kind of sin which points to the base character of the one who does it. 
Ibn al-‘Arabi said: “Muhammad (peace and blessings of Allah be upon him) was protected by his Lord against committing any sin, whether during the Jaahiliyyah or afterwards, as an honour from Allah, so that he might occupy a great and lofty status. So throughout his life he was protected from sin. 
Al-Nawawi (may Allah have mercy on him) said: 
You should note that the scholars differed as to whether the Prophets could commit sin. Al-Qaadi summed up the main points of this issue when he said… with regard to sin, there is no dispute that the Prophets were protected from committing any major sin. Similarly there was no dispute that they were protected from minor sins which reflect badly on a person and lower his status and damage his character. But they differed as to whether the Prophets could commit other kinds of minor sins. Most of the fuqaha’, hadeeth scholars and scholars of kalaam among the earlier and later generations are of the view that they could have committed such minor sins. They quote as evidence the apparent meaning of the Quran and reports. Some of the fuqaha’ and scholars of kalaam among our imams are of the view that they were protected from minor sins just as they were protected from major sins, and that the position of Prophethood means that the Prophets were far above committing sin or deliberately going against Allah’s commands.  They commented on verses and ahaadeeth which refer to mistakes on the part of the Prophet (peace and blessings of Allah be upon him) and they interpreted them in a manner other than the apparent meaning. They said that these verses and ahaadeeth refer to incidents where a Prophet based his actions on his own reasoning, or he did what he did out of forgetfulness, or with permission from Allah in cases where he feared that he would be blamed for it, or there might be mistakes that they made    before Prophethood. This last view is the one which is correct. These are the words of al-Qaadi ‘Iyaad, may Allah have mercy on him. And Allah knows best. 
Out of respect towards the position of Prophethood, the scholars said that whoever slanders any of the Prophets is a kaafir and must be executed. 
Shaykh al-Islam Ibn Taymiyah said in Majma’ al-Fataawa, 35/123 
The imams are agreed that whoever insults a Prophet should be executed. And an accusation of adultery is the worst kind of insult. See also Question no. 22809. 
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni, 12/405 
Whoever slanders the mother of the Prophet (peace and blessings of Allah be upon him) is to be executed even if he repents, whether he is a Muslim or a kaafir, but if he repents then his repentance will be accepted by Allah, but the sentence of execution will not be waived because of his repentance, because of the rights of the Prophet (peace and blessings of Allah be upon him). 
Then he said: 
The ruling concerning slander of the Prophet (peace and blessings of Allah be upon him) is like the ruling on slander of his mother, because slandering his mother brings a sentence of execution because it is a slander against the Prophet (peace and blessings of Allah be upon him) and against his lineage. 
And Allah knows best. 
See Zaad al-Ma’aad, 1/77; al-Seerah al-Nabawiyyah by Dr. Akram Diya’ al-‘Umari, 1/112-114; al-Seerah al-Nabawiyyah by Dr. Mahdi Rizq-Allah, p. 132; Af’aal al-Rasool (peace and blessings of Allah be upon him) by Dr. Muhammad Sulaymaan al-Ashqar, 1/139-165; Ahkaam al-Quran al-Kareem, 3/576.