From Aeon (hat tip: Tom MacMaster):
Islam spread through the Christian world via the bedroom
There are few transformations in world history more profound than the conversion of the peoples of the Middle East to Islam. Starting in the early Middle Ages, the process stretched across centuries and was influenced by factors as varied as conquest, diplomacy, conviction, self-interest and coercion. There is one factor, however, that is largely forgotten but which played a fundamental role in the emergence of a distinctively Islamic society: mixed unions between Muslims and non-Muslims.
For much of the early Islamic period, the mingling of Muslims and non-Muslims was largely predicated on a basic imbalance of power: Muslims formed an elite ruling minority, which tended to exploit the resources of the conquered peoples – reproductive and otherwise – to grow in size and put down roots within local populations. Seen in this light, forced conversion was far less a factor in long-term religious change than practices such as intermarriage and concubinage.
The rules governing religiously mixed families crystallised fairly early, at least on the Muslim side. The Quran allows Muslim men to marry up to four women, including ‘People of the Book’, that is, Jews and Christians. Muslim women, however, were not permitted to marry non-Muslim men and, judging from the historical evidence, this prohibition seems to have stuck. Underlying the injunction was the understanding that marriage was a form of female enslavement: if a woman was bound to her husband as a slave is to her master, she could not be subordinate to an infidel.