Unfortunately, faesce is correct on two accounts. First, he's correct that this is an issue that men often face with their girlfriends. Second, ultimately faesce's option may be the one you have to choose. If she's too dense to even try to understand that all men look at porn, then hide the evidence of your porn from her.

Also, if you're not her first boyfriend, and she thinks you're different from her previous boyfriends, then that says something about those other guys' initiative and ability to hide their porn...

On the other hand, her anger with you might be more over the fact that you are leaving evidence of your porn out in the open. In that case, it is reasonable that she requests for you to keep evidence of your porn viewing habits out of her sight. However, it sounds like more than that.

If you do want to try to make your girlfriend understand, I'm sure that Dan Savage has covered this topic somewhere in one of his Savage Love columns. Try perusing them for some advice or even use it as a reference for why your porn viewing is okay (if she'd buy his arguments).