"Also look up slavery in Islam."
Er . . . OK.
It may, superficially, appear distasteful to copulate with a woman who is not a man's legal wife, but once Shariat makes something lawful, we have to accept it as lawful, whether it appeals to our taste, or not; and whether we know its underlying wisdom or not. It is necessary for a Muslim to be acquainted with the laws of Shariat, but it is not necessary for him to delve into each law in order to find the underlying wisdom of these laws because knowledge of the wisdom of some of the laws may be beyond his puny comprehension. Allah Ta'ala has said in the Holy Quran: “Wa maa ooteetum min al-ilm illaa qaleelan” which means, more or less, that, "You have been given a very small portion of knowledge”. Hence, if a person fails to comprehend the underlying wisdom of any law of Shariat, he cannot regard it as a fault of Shariat (Allah forbid), on the contrary, it is the fault of his own perception and lack of understanding, because no law of Shariat is contradictory to wisdom.
the wisdom underlying the permission granted by Shariat to copulate with a slave woman is as follows: The LEGAL possession that a Muslim receives over a slave woman from the “Ameerul-Mu'mineen” (the Islamic Head of State) gives him legal credence to have coition with the slave woman in his possession, just as the marriage ceremony gives him legal credence to have coition with his wife. In other words, this LEGAL POSSESSION is, in effect, a SUBSTITUTE of the MARRIAGE CEREMONY. A free woman cannot be 'possessed', bought or sold like other possessions; therefore Shariat instituted a 'marriage ceremony' in which affirmation and consent takes place, which gives a man the right to copulate with her. On the other hand, a slave girl can be possessed and even bought and sold, thus, this right of possession, substituting as a marriage ceremony, entitles the owner to copulate with her. A similar example can be found in the slaughtering of animals; that after a formal slaughtering process, in which the words, “Bismillahi Allahu Akbar” are recited, goats, cows, etc.; become “Halaal” and lawful for consumption, whereas fish becomes “Halaal” merely through 'possession' which substitutes for the slaughtering.