I personally don't have a problem with porn. I like to watch it, and so does my husband. sometimes we watch together. I.M.O. if you aren't having online sex with someone, then you aren't cheating. Obviously your gf thinks differently. You lied to her, and I would think that she's more upset about the deceit than the actual porn. You need to work this out with her. Have a sit down and try to discover exactly what about it makes her so upset. Perhaps you can educate her about it. Tell her what it does for you. See if she would be willing to watch it with you. You both need to agree, and then stick by your agreement.
Here's a funny story for you. I used to be very against strip bars. I told my dh that if there were strippers at his bachelor party, to forget the wedding. Well, obviously there were strippers at his bachelors party ad nobody told me. About 2 years later we were on vacation and his friend took us to a strip bar. At first I was pissed off, then as I sat there and had a couple of cocktails, I discovered that it was a musing and fun. I had a couple more cocktails and had a great time. Our evening almost ended inme getting a tattoo, but I was too drunk and the tattoo place refused to do it until the next day (thank god!). The next day he told me about his bachelor party, and I wasn't mad at all. I realized I was just being stupid. (I am not saying your gf is being stupid, just me). Now we go to strip clubs every once in a while together. For my 12th anniversary, a bunch of my friends went there with us and I got a stage dance. That was funny. The girl kept talking to me and when I was done the guys with us asked what we were talking about and I said "hair care products", which was the truth, but they never believed me.
Anyway, my point is that sometimes we can change even our deepest convictions when we are shown the truth. Good luck.