One reason that the Islamic world views us hostilely is that they believe that our culture is thoroughly dissolute. Considering that their view of our culture comes almost entirely from Hollywood and television, that's not surprising. (The great irony, of course, is that those who most raise the ire of the Islamic world, and who most need to be defended against the violence that results, are among the least willing to support that defense.) Early on, Osama bin Laden's main objective was complete isolation of the Middle East from the West, cutting off the corrupting influence of the West.
It's not surprising that Islam views homosexuality as wrong (so does Christianity and Judaism). However, for the vast majority of the Middle East, homosexuality is not just sinful, it is vile and punishable by death. Imagine, then, their perspective on something like homosexual marriage, and the country that would permit it. A Federal Marriage Amendment, from their view, would show that the US is willing to draw the line somewhere, and share their moral beliefs at least to some degree.
The US should not decide its internal policy according to the views of the rest of the world. Frankly, our policies have succeeded better than those of the Middle East (obviously), or even Europe. However, it's interesting to note the ripples that flow from our decisions. (Old Europe would hate it, obviously, but I'll admit I'm not particularly concerned about that.)