A 12 year old girl in Saudi Arabia is seeking a divorce from her 80 year old husband. (Girls as young as 8 are regularly sold to and raped all over the Middle East and North Africa- 77 percent in Niger.)
Saudi clerics and judges are defending the marriage on the basis that the Prophet Muhammad married (and had sex with) a nine year old girl.
Apologists in the Islamic and Western world will inevitably argue that selling young girls into sexual bondage may be something that was acceptable 1,400 years ago, but not today.
That this response is offered as a defense of the Quran/Bible/Torah reveals the fatal flaw in their logic:
By what standard is something not morally acceptable today that was acceptable earlier? If a superior standard of morality exists by which we can judge human action, what is the point of “holy books?” If the holy books sanction slavery, rape, murder, genocide (as they all do), then why would anyone claim that they are a source of moral authority?
Even if you disagree with a single instruction of the Bible (such as the command to kill any bride who is not a virgin, or any child who disrespects his parents) then you acknowledge that there exists a superior standard by which to judge moral action, and there is no need to rely on a bunch of primitive, ancient, barbaric fairy tales.