The rules seem to be considered as a fatwa of IS. However, the contents of the document are contradictive as well as uncivilized.
International Business Times: 442Isis sex slave manual reveals Daesh's sordid rules on how to rape female captives4813046
If this report is true, it suggests not only the mercilessness of IS but also childishness. Some articles in the fatwa seems absurd, apart from these unethical sense. Although there are many ideologies in the world, approval of sex slaves should never be allowed, at least in my opinion.
Sex slavery itself is not an ancient habit. In Japan at the time of the WWII, several Korean women were considered to be engaged in sexual services for Japanese military as “comfort women,” though there is controversy about the degree of coerciveness on it. In other countries similar schemes were likely to exist.
However, in the 21st century, the activity of IS is far from understandable. Not to abuse captives even in the war is a common law of the modern world. It was established before the WWII, though often has been ignored.
Gender equality is one of the modern ideology. I am not sure how this rule is reasonable. But if I violate this idea, I would never be able to make a complaint when I am abused.