Re: ISIL and the enslavement of Women
You are talking about two different subjects here. Slavery in Islam, and sex with female slaves.
Let me first make it clear about sex with female slaves or sex slaves as you call it.
Quran does prohibit sex outside marriage with anyone including female slaves.
And whoever of you cannot afford to marry the independent believing women, then from those committed to you by oath of the believing maidens. God is more aware of your faith, you are to each other (i.e. equal). You shall marry them with the permission of their guardians and give them their dowries in kindness; to be protected, not for illicit sex or taking lovers. If they become protected, then any of them who comes with lewdness shall have half the punishment of what is for the independent women. This is for those who fear hardship from among you. But if you are patient it is better for you, and God is Forgiver, Merciful.} Surat an-Nisa’ 4:25*
Now as to your question why Quran and Hadith didn’t ban slavery.
yes, Quran and Hadith didn’t ban slavery outright but the rules and regulations that the Quran and the Prophet( pbuh) brought to us in regard to the wellbeing and rights of slaves eventually led to the abolishment of slavery.