The female captives, in compliance with Quranic sanctions and prophet traditions, were used as sex slaves by the Muslim masters. Therefore, they did not only add to the growing Muslim population but also became valuable tools for expanding the Muslim populace through procreation.
When those women, especially the ones of childbearing age, were taken away, the Hindu men, who had fled, came back to find that their women and children were gone. As a result, they did not have sufficient partners for procreation. That means, where Muslims made a successful assault, procreation in the Hindu community dropped sharply. On the other hand, the few thousand Muslim soldiers who came to India with Muhammad bin Qasim had plenty of sex partners for reproduction to the capacity.
Therefore, whenever Muslims made successful inroads, they reduced the Hindu population directly by slaughtering the men in larger numbers and taking away women and children as captives. It indirectly reduced the Hindu populace by rendering the remnant Hindu men unprocreative by depriving them of childbearing female partners. Since those women became the vehicle for breeding Muslim offspring instead, the result was a reduction of the Hindu populace, maintained by the toiling of the vanquished Hindus, subjected to grinding taxes. This is roughly the same protocol which Prophet Muhammad had applied to the Jews of Banu Qurayza and Khaybar.
(M. A. Khan. (2009) Islamic Jihad: A Legacy of Forced Conversion, Imperialism, and Slavery. iUniverse, Bloomington, IN, p. 102).
References: Milton, G. (2004). White Gold. Hodder & Stoughton, London.
