Actually the Koran forbıds the enslavement of Chrıstıans and Jews, as well as Muslıms.
But it does appear to permit the rape of "slaves".
Here is a reference permitting to rape captive women:
Also (prohibited are) women already married, except those whom your right hands possess: Thus hath God ordained (Prohibitions) against you: Except for these, all others are lawful, provided ye seek (them in marriage) with gifts from your property, - desiring chastity, not lust, seeing that ye derive benefit from them, give them their dowers (at least) as prescribed; but if, after a dower is prescribed, agree Mutually (to vary it), there is no blame on you, and God is All-knowing, All-wise. S. 4:24 Y. Ali
The above passage emphatically allows for the raping of women that are taken captive, even if these captives happened to be married!
It did not remain an abstract theoretical right, but was readily put into practice by the Muslim jihadists:
Abu Sirma said to Abu Sa'id al Khadri (Allah he pleased with him): O Abu Sa'id, did you hear Allah's Messenger (may peace be upon him) mentioning al-'azl? He said: Yes, and added: We went out with Allah's Messenger (may peace be upon him) on the expedition to the Bi'l-Mustaliq and took captive some excellent Arab women; and we desired them, for we were suffering from the absence of our wives, (but at the same time) we also desired ransom for them. So we decided to have sexual intercourse with them but by observing 'azl (Withdrawing the male sexual organ before emission of semen to avoid conception). But we said: We are doing an act whereas Allah's Messenger is amongst us; why not ask him? So we asked Allah's Messenger (may peace be upon him), and he said: It does not matter if you do not do it, for every soul that is to be born up to the Day of Resurrection will be born. (Sahih Muslim, Book 008, Number 3371)
Abu Said al-Khudri said: The apostle of Allah (may peace be upon him) sent a military expedition to Awtas on the occasion of the battle of Hunain. They met their enemy and fought with them. They defeated them and took them captives. Some of the Companions of the Apostle of Allah (may peace be upon him) were reluctant to have intercourse with the female captives in the presence of their husbands who were unbelievers. So Allah, the Exalted, sent down the Quranic verse, ‘And all married women (are forbidden) unto you save those (captives) whom your right hands possess’. That is to say, they are lawful for them when they complete their waiting period. (Sunan Abu Dawud, Volume 2, Number 2150)
Ibn Kathir wrote:
<except those whom your right hands possess>
except those women whom you acquire through war, for you are allowed such women after making sure they are not pregnant.
Imam Ahmad recorded that Abu Sa’id Al-Khudri said, "We captured some women from the area of Awtas who were already married, and we disliked having sexual relations with them because they already had husbands. So, we asked the Prophet about this matter, and this Ayah was revealed…
<Also (forbidden are) women already married, except those whom your right hands possess>.
Consequently, we had sexual relations with these women." This is the wording collected by AT-Tirmidhi, An-Nasa’i, Ibn Jarir and Muslim in his Sahih. (Tafsir Ibn Kathir (Abridged) Volume 2, Parts 3, 4 & 5 (Surat Al-Baqarah, Verse 253, to Surat An-Nisa, Verse 147), abridged by a group of scholars under the supervision of Shaykh Safiur-Rahman Al-Mubarakpuri [Darussalam Publishers & Distributors, Riyadh, Houston, New York, Lahore; First edition, March 2000], p. 422
http://www.answering-islam.org/Shamoun/women_in_islam3.htm
(Evangelist Christian website)