The marriage ceremony is expensive - especially in the Middle East. And in countries like Egypt have low economic growth and huge inflation in marriage prices, so males have to work long and hard into their 30es to get married. And they can't just get a girlfriend and marry her in a inexpensive ceremony with only a civil unions - the religious leaders made sure its illegal (they have a interest in preserving the expensive traditions, you see).

Most men in the Middle East can hardly afford a single wife. Only really, really rich men have harems.

Of course the reasons are way more complex.

I'll recommend this excellent article from Fareed Zakaria: The Politics of Rage: Why Do They Hate Us? http://fareedzakaria.com/articles/newsweek/101501_why.html

It's not very common in practice though. Not enough to create demographic problems.