Why are Muslims homophobic?

I knew a girl from Lebanon, a highly Christan state, and she was realllly homophobic (ironically, she'd married her cousin). Then I was reading New York magazine and I saw that in interviews with people of Beirut, a lot of anti gay marriage sentiment was expressed. So that made sense....but I then wondered about other middle eastern countries...like Afghanistan is really against gays - but why? Is that a tenet of Islam?