Culturally, lots of sexual deprivation among young men locally, due to the fact that they aren't allowed to generally interact with females, and the populace practices polygamy, along with strict punishment for extra-marital male-female relations. So fewer, older, wealthier males take multiple females a piece, and younger, poorer, angrier males are left with none (it is worth noting that a second result of this is widespread homosexuality in the Arab world, and pederasty in Central Asia). Among the foreign fighter flow, that's simply who is attracted to the Call - young men who are generally outcasts or who don't fit in well with society in the West. Generally, these are guys who have never held a girls' hand, and now they can pick up Yazidi females for $15 on the local market and finally relieve that pent-up frustration, sexual desire, and (sadly) anger. It's awful for the women (many of whom are still children) involved.
