Why Prophet (SWS) Didn't Marry Maria Qibtia
If you judge a past practice with the present norms, it is unacceptable, but this is the wrong way to judge it. The Quran did not make it prohibited, but rather allowed it, as it was the custom. What the prophet did, was within the premises of Islam. Slaves were not forced to remain slaves. In the law of Mukatabat, they were allowed to buy their freedom by paying their price to their masters, and to help them, God prescribed a head of Riqab in the zakat fund to liberate the slaves who wanted to be free. But despite this law, slaves did not choose to go free. Because, they were enjoying livelihood, residence, and affiliation with a respectable person of the society. This was the situation. Now the situation is different.
Please also refer to the video below from 34:39 to 39:14 https://youtu.be/J_XuFIMcfT4?t=2079