This is interesting:
Question - 13737
In Ar-Raheeo Al Makhtum (The Sealed Nectar) the author says in the section called "The Prophet Household" that the Prophet (S.A.W.) had four concubines
1. Why is it that having concubines is not haram?
2. Can other muslims have concubines?.
Answer
Praise be to Allaah.
Allaah is Most Wise in the laws that He decrees for His slaves, but this wisdom is only apparent to those who seek it and believe that Allaah is All Wise, and looks at the interests that are served by the laws of Allaah which no man can see unless he ponders the matter, especially when there are those who attack those laws because they go against what they think are right and wise.
With regard to your question about it being permissible for a master to be intimate with his slave woman, the answer is that that is because Allaah has permitted it. Allaah says (interpretation of the meaning):
“And those who guard their chastity (i.e. private parts, from illegal sexual acts)
6. Except from their wives or (the slaves) that their right hands possess, for then, they are free from blame”
What is the ruling on intimacy with slave women? - Islam Question & Answer