Consider the following hadith:
"We [Muhammad’s merry men] went out with Allah's messenger on the expedition to the 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 [i.e., Muhammad] is amongst us; why not ask him?" So we asked Allah's messenger 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, Vol. 2, #3371)
Tellingly, the role model of the Muslims, Muhammad, does not exclaim:
“DO NOT FORNICATE WITH THE CAPTIVES!”
Nor does he exclaim to the married men: “DO NOT COMMIT ADULTERY WITH THE CAPTIVES!!”
Finally, we note that he does not yell: “DO NOT RAPE THE CAPTIVES!!!”
So given that the role model of the Muslims, Muhammad, had no problem with female captives being raped, can we truly be surprised that the Islamic State has no problem with the rape of captives either?