Qur'an (33:50) - "O Prophet! We have made lawful to thee thy wives to whom thou hast paid their dowers; and those (slaves) whom thy right hand possesses out of the prisoners of war whom Allah has assigned to thee" This is a special command that Muhammad handed down to himself, allowing himself virtually unlimited sex. Others are limited to four wives, but may also have sex with an unlimited number of slaves, as the following verse make clear:
Qur'an (23:5-6) - "..who abstain from sex, except with those joined to them in the marriage bond, or (the captives) whom their right hands possess..." This verse allows the slave-owner to have sex with his slaves. See also Qur'an (70:29-30).
Qur'an (4:24) - "And all married women (are forbidden unto you) save those (captives) whom your right hands possess." Even sex with married slaves is permissible.
Qur'an (8:69) - "But (now) enjoy what ye took in war, lawful and good" A reference to war booty, of which slaves were a part. The Muslim slave master may enjoy his "catch" because (according to verse 71) "Allah gave you mastery over them."
Qur'an (24:32) - "And marry those among you who are single and those who are fit among your male slaves and your female slaves..." Breeding slaves based on fitness.
Qur'an (2:178) - "O ye who believe! Retaliation is prescribed for you in the matter of the murdered; the freeman for the freeman, and the slave for the slave, and the female for the female." The message of this verse, which prescribes the rules of retaliation for murder, is that all humans are not created equal. The human value of a slave is less than that of a free person (and a woman's worth is also distinguished from that of a man's).
Qur'an (16:75) - "Allah sets forth the Parable (of two men: one) a slave under the dominion of another; He has no power of any sort; and (the other) a man on whom We have bestowed goodly favours from Ourselves, and he spends thereof (freely), privately and publicly: are the two equal? (By no means

praise be to Allah." Yet another confirmation that the slave is is not equal to the master. In this case it is plain that the slave owes his status to Allah's will. (According to 16:71, the owner should be careful about insulting Allah by bestowing Allah's gifts on slaves - those whom the god of Islam has not favored).