Among the verses abused by some oppressive traditional Muslims are those dealing with having intimate relations with those who are “rightfully theirs” or “legally theirs” (in Arabic, “Ma malakat aymanukum”).
[23:5-7] And they maintain their chastity. Only with their spouses, or those who are rightfully theirs, do they have sexual relations; they are not to be blamed. Those who transgress these limits are the transgressors.
[70:29-31] They keep their chastity. (They have relations) only with their spouses, or what is legally theirs―anyone who transgresses these limits is a sinner ...
The verses referring to “legally (rightfully) theirs” deal with “common law marriages”. In some societies, there is an accepted custom of a husband and wife committed together for life without a formal marriage and dowry. How is this different from a Quranic marriage? In addition to the commitment to want to be together for life, Quranically, a marriage requires mutual attraction and dowry (4:24). Verses like 23:6 and 70:30 account for the different types of marriages in different cultures around the world.
Unfortunately, some misguided so-called "Islamic scholars" misinterpret the verses to mean that sexual relations without marriage with a captive or slave is legitimatized in the Quran. They mistranslate "rightfully theirs" as "what the right hand possesses" to come up with an oppressive meaning that goes against what the Quran teaches. The verses could not possibly refer to having “female sexual slaves” for a number of reasons.
Taking a woman captive and forcing her into sexual relations is oppression and rape. Equality of women, and protection of women against oppression is emphasized in the Quran. For example, chapter 4 of Quran is titled “Women” (Al-Nesã’) and the sura's theme is “protection of women.” Oppression is strongly condemned throughout the Quran (2:191, 2:193, 2:217, 10:85-86, 28:76) and God calls it “worse than murder” (2:191, 2:217). God is on the side of the oppressed against the aggressors (4:75, 7:137, 8:26, 28:5, 54:10-11).
God tells us in the Quran that all humans are children of Adam (4:1, 7:26-27, 7:35, 17:70, 36:60), i.e., they have the same ancestry and source of genetic material. Essentially, we are all equal as humans.
[4:1] O people, observe your Lord; the One who created you from one being, and created from it its mate, then spread from the two many men and women. You shall regard GOD, by whom you swear, and regard the parents. GOD is watching over you.
If we are all children of Adam, it is not logical that one person can be forced into slavery by another. Slavery (of men and women) is discouraged in Quran, and God encourages believers to use every opportunity to free the slaves (2:177, 4:92, 5:89, 9:60, 58:3, 90:13, etc.).
Men and women are equal in belief (33:35), and the soul (the real person) has no gender. The only criterion to distinguish one person from another is righteousness (49:13). God tells us to treat all His servants in the best possible manner (17:53), with kindness and humility (52:26), especially our partners (4:19). None of this is consistent with the traditional scholars’ interpretations supporting enslavement and oppression of women.
The Quran clearly says you must free a believing slave before marriage and give them their due dowry (4:25). It is not sufficient to free them, they also have a right to dowry (i.e., one cannot take advantage of their being slaves to treat them less than equitably). God reminds us that slaves and free people are equal in terms of belief. They are entitled to the same financial protection and respect as other women (4:24).
[4:25] Those among you who cannot afford to marry free believing women, may marry believing slave women. GOD knows best about your belief, and you are equal to one another, as far as belief is concerned. You shall obtain permission from their guardians before you marry them, and pay them their due dowry equitably. …
The Quran makes it clear that we must maintain our chastity (5:5, 23:5, 24:30-31, 70:29) and avoid giving in to our lusts (4:27, 79:40). It is illogical to conclude that a barbaric interpretation of “Ma malakat aymanukum” (i.e., forcing women to become “sex slaves”) is consistent with Quranic principles of freedom, non-oppression keeping one's chastity and enjoining the self from lusts.
In conclusion, having intimate relations outside of marriage (whether a Quranic or a common-law marriage) is a gross offense and completely unQuranic. The verses about having relations with “spouses” or “legally/rightfully” theirs (Ma malakat aymanukum) refer to practices known as common-law marriages.