I am a Muslim and came around a discussion with an atheist friend. He asked me this question, that why Prophet Muhammad had relationship with Maria Qibti, though they were not married. They had a son also out of this relationship, his name was Ibrahim. What is the actual story and how Muslim scholars react to it?
Well the Lady Maria the Coptic -may Allah be pleased with her- was a so called mother of child (son) after giving birth to her son Ibrahim and with that she was free and therefore haram to marry after the death of the Messenger of Allah peace be upon him.
Before that she was a women who came in the possession of the Messenger of Allah peace be upon him according to the Verse 50 in Surah Al-Ahzab (33).
This is one of the Verses in the Quran which show that one could have a sexual relationship with his slave girl, without the marriage rules we know for a free woman/girl! But there are some rules claimed in fiqh books about these kinds of relationships (concubinage).
The Prophet (peace and blessings of Allaah be upon him) did not marry Mariyah al-Qibtiyyah, rather she was a Concubine who was given to him by al-Muqawqis, the ruler of Egypt. That took place after the treaty of al-Hudaybiyah. Mariyah al-Qibtiyyah was a Christian, then she became Muslim (may Allaah be pleased with her). —Ibn Saad, The Life of Prophet
The Messenger of Allaah (peace and blessings of Allaah be upon him) lodged her – meaning Mariyah al-Qibtiyyah and her sister – with Umm Sulaym bint Milhaan, and the Messenger of Allaah (SallAllahu 'alayHi waSallam) entered upon them and told them about Islam. He took Mariyah as a concubine and moved her to some property of his in al-‘Awaali… and she became a good Muslim. —Al-Tabaqaat al-Kubra, 1/134-135
The Prophet (peace and blessings of Allaah be upon him) had four concubines, one of whom was Mariyah. —Ibn al-Qayyim , Biography
You can read more about her Biography here or even in wikipedia (Maria al-Qibtiyya مارية القبطية)
You can also find a Fatwa that explains why she isn't a um al mu'minyn!
By the way the beginning of Surat at-Tahrim (Surah 66) was sent down "because of her"!
This is briefly what is said about Maria the Cobtic in special!
At last one should say that Islam accepted slavery but didn't -really- encourage it, so you couldn't change a free man/woman/child into slave as it happens in medieval Europe (where for example debtors turned into slaves of their donor). And the kafara of many mistakes was/is to free a slave. Therefore one could say that Islam was for a bit by bit abolition of slavery!
Three kinds of slavery were accepted AFAIK: a born slave, a bought slave and a prisoner of war! All of them shouldn't exist by now because
In a sahih Hadith you'll find the following statement of our Messenger (Peace be upon him):
"There are three (1) whose adversary I shall be on the Day of Resurrection: ...
a man who has sold a free man (2) and has consumed the price ...".
(1) i.e. types of men.
(2) i.e. a man who has made a slave of another and has sold him.