Like many other evils in the world, unrestrained mistreatment of captured enemies has been around throughout human history. So I don’t blame Islam for starting it. However, more than any other major religion their scripture certainly condones it the most. The reason is obvious. The “prophet” who wrote their scripture was a horrible wicked man.
It is not merely a question of Islam condoning rape and mass rape. Their religion prohibits what it claims is evil, and encourages what it claims to be good. Clearly it encourages the mass rape of all female captives, unlike every other legal system. To be a good Muslim, you are practically required to rape them. Historically the Muslims have made sex slaves of tens of thousands of women and returned some decades later with the offspring having formed a new Muslim army.
Since Muslims consider their religion's founder to be the Seal of the Prophets, they cannot change their laws. It is their law, unchangeable and in action, that is so offensive to modern civilized minds.