So what's the reason behind what's going on in Afghanistan? It seems the situation is similar to the one in Yemen which we discussed here. The reason being poverty!
Prepubescent sex is not allowed in Islam. To learn a bit about why prepubescent sex and abuse can take place in rural Afghanistan and rural Yemen please see this post about Yemeni child brides where an Islamophobe was wrongly blaming Islam for the problem in Yemen:
Learn about Islam:http://www.thedeenshow.com