Hm.. Well, RealisticIntuition, honestly I don't think she did it because she has a worse addiction than you or anything like that. I think maybe she did it to get you to understand the pain she feels when you do relapse, especially after the night of the best sex in her life. Maybe she feels at fault for causing you to relapse, too, because she can't express herself sexually to you anymore because you are at risk or relapse and it scares her to death. And maybe it just hurt her so much that she couldn't take it, and that's not even your fault.. I'm sure she is getting through it. She might need you, though, to get through it together.
And maybe, just maybe, she hates porn and never even thinks to watch it and only did this to (dysfunctionally) express herself to you. She might have just lashed out because she didn't know how to deal with what she was feeling otherwise. It was wrong, nonetheless, and she takes full accountability for it.
Porn, perhaps, might be getting in between you two, and I think you should try and bear with her feelings a little bit because she might be going through an emotional time right now in her life. I am willing to bet that she loves to pieces and never meant to hurt you and that she is sorry for reacting this way.
Talk to eachother in person about this because you just might found out that she loves you so much and really will never do something like this again.. and also, she might tell you that she accepts you, every single part of you, deep, shallow, dark or light.. and that she loves you SO MUCH and is ready to move past this issue and grow into a deeper connection with you and support you on your journey to become a better man because she actually wants to inspire you to be the best you can be, through the hard, mad times, and the fun times.
And if it doesn't work out with her, you can always hit me up because I think you're amazing. Sorry you had to go through all this. I wish you the best of luck on your journey. <3
P.S. She thought of you the entire time while she had the porn on.. and closed her eyes the whole time because she couldn't stand looking at the act of sex between others, facing that reality of the thing that is destroying her relationship right now on that monitor.. and she also does value yours and her sacred bond so strongly that she would never fantasize about anyone or anything other than her man. It's no excuse for this girl, but I just felt like I should add it in. She never wants to lose you. For the love of god, hold on to her tight because she fucking loves you.