I keep typing this and can't quite word it right but here goes anyway.
OP, you say you are shocked by porn and feel it degrades women and sends bad messages. (I agree with you, especially when talking about the majority of internet porn)
The trouble is that porn is so normalised in our society now and in her generation, that if you try to talk to her from this angle, she's most likely to write it off as you being out of touch, than listen to what you have to say, because the overwhelming majority view (as seen on this thread!) is that porn is fine, no big deal, it's normal.
And of course there's the added complication that she would most likely be mortified if you brought it up and just say anything to get the subject closed and over with. So you will need to bring it up, if possible, without intimating that you are aware of the fact she has been looking at it.
You would definitely find some interesting/helpful articles or books (or threads!) mentioned in the feminism section on mumsnet, I think. Maybe some which you could share with your DD. Even if you don't consider yourself a feminist it sounds as though you have some feminist principles, so you would probably find it interesting too.
I don't think banning it is going to help, it will just cause embarrassment (and she will probably see it or gain these messages from elsewhere anyway) - so you need to counter it with some other views, and not necessarily directly from you.