The Quran makes it very clear it is permitted for men to have sex with "those whom your right hands possess" or in other words - women used as sex slaves, outside marriage. Slavery itself is unacceptable because slaves are obliged to obey their masters, being considered their "possessions". With this in mind, it is very clear that using slave women for sex effectively amounts to rape.
4:24 - Also (prohibited are) women already married, except those whom your right hands possess: Thus hath Allah 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 Allah is All-knowing, All-wise.
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,- for (in their case) they are free from blame.
70:27-30 - And those who are fearful of the punishment of their Lord, indeed, the punishment of their Lord is not that from which one is safe - And those who guard their private parts, Save with their wives and those whom their right hands possess, for thus they are not blameworthy.
The hadiths describe this clearly, referencing Muhammad's words:
1). Permission to use female captives for sex slavery, even if they are married:
Sunan Abu Dawud 11:2150 - Abu Said al-Khudri said: "The apostle of Allah 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 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." [The Quran verse is 4:24]
2). Muhammad advises men to stop withdrawing their penis during the sexual act (coitus interruptus) and instead complete the sexual act:
Sahih Bukhari 5:59:459 - Narrated Ibn Muhairiz: "I entered the mosque and saw Abu Khudri and sat beside him and asked him about coitus interruptus. Abu said, "We went out with Allah's messenger for the Ghazwa (attack upon) Banu Mustaliq and we received captives from among the Arab captives and we desired women and celibacy became hard on us and we loved to do coitus interruptus. So when we intended to do coitus interruptus we said "How can we do coitus interruptus without asking Allah's messenger while he is present among us?" We asked (him) about it and he said "It is better for you not to do so, for if any soul (till the Day of Resurrection) is predestined to exist, it will exist."
3). Muhammad supports Ali who obtained a slave girl from "khumus" or war-booty:
Sahih Bukhari 5:59:637 - Narrated Buraida: The prophet sent Ali to Khalid to bring the Khumus ([one fifth] of the booty) and I hated Ali, and Ali had taken a bath (after a sexual act with a slave girl from the Khumus). I said to Khalid, "Don't you see this (i.e. Ali)?" When we reached the prophet I mentioned that to him. He said, "O Buraida! Do you hate Ali?" I said, "Yes" He said, "Do you hate him, for he deserves more than that from the Khumus."
CONCLUSION: Needless to say this is absolutely unacceptable. One might wonder why would a religion like Islam which normally strictly forbids sex outside marriage allows for this despicable practice. But when we look at things closely, sex in Islam follows a pattern:
1). Men are allowed to marry upto four wives while women are allowed only one husband.
2). Men who lead normal lives are restrained by the strict rules prohibiting sex outside marriage.
3). Men who take part in battles can obtain any number female captives who can be used for sex.
4). Men who are martyred in battles are promised paradise and extreme sexual pleasure in afterlife.
What do we see here? Islam was a religion made for men which entices men to take part in battles and get martyred to get the ultimate sexual pleasures on earth and paradise. If we cannot see through this, can we see through anything? It is good that Muslims don't mostly follow all of this today, but that doesn't change the fact that when the Islamic fundamentalist groups do it, their actions are well-justified scripturally.