Both the Islamic State and Taliban were formed by soldiers and insurgents left over from military invasions of the Middle East. The former came to prominence thanks to a lot of Bathists in the aftermath of the Iraq invasion - many were officers during Saddam's regime - and the latter by the mujahideen, who fought the Soviet invasion of Afghanistan and had plenty of military hardware left over to form a powerful army afterwards.

The pressure from polygamy might have contributed to the individual motivations for some people to join those groups, but it was probably far from a significant factor in the geopolitics.