and this, which lists in more detail the women with whom Muhammad himself can have sex. (Wasn't it good of God to make a special revelation telling humanity forevermore whom the Prophet could bonk?):
O Prophet! We have made lawful to thee thy wives to whom thou hast paid their dowers; and those whom thy right hand possesses out of the prisoners of war whom Allah has assigned to thee; and daughters of thy paternal uncles and aunts, and daughters of thy maternal uncles and aunts, who migrated (from Makka) with thee; and any believing woman who dedicates her soul to the Prophet if the Prophet wishes to wed her;- this only for thee, and not for the Believers (at large); We know what We have appointed for them as to their wives and the captives whom their right hands possess;- in order that there should be no difficulty for thee. And Allah is Oft- Forgiving, Most Merciful. 33:50
it seems that intercourse with slaves was probably considered a method of contraceptive sexual enjoyment through coitus interruptus (`azl), since the slave owner could practice `azl without prior permission from his slave mate while he could not do so with his free wife without prior permission from her. And if the contraception intended by this `azl failed and the slave woman still bore a child from her master, her child was automatically freed and obtained a son or daughter's rights including inheritance. In addition, the mother herself could no longer be sold and was freed upon the owner's death. From the slave's perspective, the above scenario could have formed an accepted kind of lawful gamble from which she stood to gain much more than to lose. This could be problematized with the claim that "the cost of freedom is therefore rape" but such is just an inflammatory rephrasing of the truism that the cost of a war captive's life is her imprisonment; emancipation from which is a dramatically enhanced possibility in the above scenario.
(Thanks to Rational Islam? reader, Martin for the suggestion for this post)