Having been a daily consumer of a vast amount of news reporting for half a century or so, I'm not easily horrified, but an article in the New York Times recently pushed my horror button about as hard as it's ever been pushed. You should read the entire article, but, in brief, it describes how Islamist State has made the rape of Yazidi girls as young as 12 into a religious virtue.
First, the young warrior buys his sex slave from IS higher-ups. (The Koran, it seems, fully endorses the enslavement of "heretics," especially apparent polytheists like the Yazidi.) He is obliged to recite specific prayers before he rapes the girl, and others afterwards. Thus, not only is the rape not sinful, it is transformed into a kind of religious duty.
Soldiers have been raping captured women and girls for millennia. Often, rape is seen as serving the military purpose of demoralizing the enemy, while adding some extra motivation for the foot soldiers putting their own lives at risk. Why do I find the religious component introduced by IS especially distasteful? I'm not sure.
I suppose that IS fighters are meant to think of themselves as "holy warriors," pure in body and mind. That sense of moral superiority might be threatened if they were left to run around raping whoever happened to be available, like your average soldier in your average war. Ritualizing the inevitable rape keeps them "pure," and worthy of service to Allah and whoever is claiming to be caliph at the moment. If they never doubt their purity, and eventual entrance into Paradise, they are better fighters.
Needless to say, selling sex slaves also generates some extra cash for the organization. Do IS leaders actually believe what they're telling the rank and file? I don't know. Maybe. Probably. Humanity is an extremely fucked-up species, and organized religion is Exhibit A.