Actually, it's been suggested that addiction to pornography causes sexual disfunction in real relationships. Usually it is the case that the man is the addict who finds that he cannot be attracted to his female partner, but it does happen the other way round too. If she is indescriminately watching pornography, but is unable to feel sexually aroused in real life, then I would really consider the possibility of a porn addiction being the root of the "dry spells." She says she doesn't have a high sex drive... that might be true. Porn may have ruined real sex for her.
It is worth finding out what caused her porn addiction. (I'm not saying this because she is a woman, I'd say the same of a man unable to function sexually who watched porn by himself when his partner was gone.)
For example, in a previous relationship my partner insisted I watch porn with him. I hated it, but he'd ask me to watch "girl on girl" action with him, and even after we broke up I kept having those images in my head... they got in the way of real relationships for a while before I broke the pattern. Was she exposed to sexual behaviour at a young age? Did a previous partner introduce her to it? Was she afraid of sex, and watched porn to demystify it? I've know people who met all of those descriptions.
But it is possible that she has a low sex drive, and is sadly addicted to porn. Porn would make her less happy with real sex, reinforce feelings of inadequacy (who can live up to those stereotypes) and ironically destroy her ability to actually engage in a healthy sexual manner with the man she loves.
I'm really sorry for both of you. I hope you can get past this... please remember as well how horribly embarassing it is for a woman to have a problem like this... many times more so than it is for a man. No wonder she hid it from you. Please don't think she doesn't love you. I imagine she's been trying to protect you.
You both need to get some help on this. It's the cruellest form of sex addiction there is... it keeps feeding an insatiable hunger, but destroys the joy of real sex.
Please do let us know what's happening... there must be other silent sufferers out there, and you never know, you might be able to help one of them. In the meantime, I'm a woman, so I can't imagine how this feels to you as a man... it must hurt so badly. It feels like an infidelity, but there's nothing you can name.
Let us know if she's open and willing to discuss her problem, and if you're able to keep your cool... it would be so easy to shout at her and get angry. I hope you can curtail this very understandable reaction, since I do think you can get past this. Both you and she deserve better.
Let her know that if she posts on this forum people will be understanding, and she'll get some help. But for the most part, look after yourself. You've just had a horrible shock. I hope you are feeling steadier soon... as someone who was involved in a relationship with a (male) sex addict I can understand how saddened and betrayed you must feel. I hope that, unlike me, you don't allow that addiction and betrayal to affect your own peace of mind. This can be fixed, if all parties are honest. You were very brave to post as you did on this forum.