This is typical Isis provocation, designed to appeal to frustrated young men while simultaneously shocking orthodox Muslim opinion. And quite predictably, Islamic scholars like Professor Abdel Fattah Alawari, dean of Islamic Theology at Al-Azhar University in Cairo, rushed to point out that Isis, in claiming that this was part of Shariah law, was deliberately misreading verses and sayings that were originally designed to end slavery.
"Slavery was the status quo when Islam came around," Alawari said. "Judaism, Christianity, Greek, Roman, and Persian civilisations all ... took the females of their enemies as sex slaves. So Islam found this abhorrent practice and worked to gradually remove it." Well, yes, but very, very gradually.
Islamic law forbids the enslavement of Muslims, but all that did was to encourage a roaring trade in the enslavement of non-Muslims that lasted for over a thousand years. And it reached a very long way: when I was growing up in Newfoundland, the easternmost part of North America, we learned in school about the "Sally Rovers", Muslim pirates from Morocco who raided villages on the Newfoundland coast for slaves until well into the 18th century.
Muslim slave raids on the Mediterranean coasts of Europe were so constant that long stretches of coastline remained largely abandoned until the 18th century.
Christianity, which spread widely among slaves in the Roman empire and did not control any government for the first three centuries of its existence, ought to have done better when it came to power, but it didn't. Slavery lasted in the eastern part of the Roman empire, Byzantium, until that finally fell to the Turks in 1452. Slavery had pretty well died out in the Christian West by the year 1000, only to be replaced by the feudal system in which most common people were reduced to serfdom. And as soon as a demand for actual slave labour re-appeared, with the European colonisation of the Americas in the 16th century, the Europeans began to buy slaves from Africa - as the Islamic empires of the Middle East and India had been doing all along.
Neither the European empires nor the great Muslim states ended slavery until the 19th century, so there is plenty of blame to go around. But there is one striking difference between the two trades. The European slavers took two or three African males for every female, because what they wanted was a work-force for commercial agriculture.
The Muslim slavers, by contrast, generally took more women than men, because there was a bigger demand for women as sex slaves than for men as warrior slaves, and practically no demand for agricultural workers. The Muslim world does have a particular history in the question of sexual slavery, and therefore a particular duty to condemn and fight against the odious doctrinal claims of the Isis fanatics.
Gwynne Dyer is an independent journalist who specialises in writing about international affairs.