What Does Porn Do?

From the Arizona Daily Wildcat (via Mandy's Sex Life) comes this op-ed on the effect of porn.

I remember the first - and only - women's studies class I took at the UA, in which I learned that pornography was the commercial manifestation of an oppressive patriarchy, an allegory of women's subjugation, exploitation and marginalization. I remember reading and hearing again something I'd read and heard many times before: that the women who choose to make a living in pornography are victims - that they are, in fact, dehumanized to such an extent that they are unable to identify themselves as victims.

It is a version of this mantra that drives many women's aversion to porn - the idea that the genre "objectifies" women (a word that is often used, but rarely defined). But in what sense are women objectified by porn?