Lianne,
I'm sorry you are dealing with this. My husband and I went throught something similar a few years back, and it was a very difficult time for both of us. Let me first say that though I am against porn, I realize not everyone is and it is not my right to make that decision for them. But, if porn is to be allowed into a marriage, that has to be a decision both participants in the marriage make and are comfortable with. In my situation it turned out that my husband was a porn addict. He had never told anyone. It was shocking to me as he was always the guy who left the bachelor party when the strippers showed up. He reacted very badly when I found out. It required a LOT of compassion, but also firmness to help him with this. From my point of view, I told him, It was not necessary for him to agree with my anti-porn stance and I was not his mother so it was not for me to keep it out of his life. I am his wife, however and will not tolerate secrets or sneaking - it's just not how I want to live. If he wants porn in his life than he can be a man and defend his views to his wife even if I don't agree.
He did not want porn in his life - he thinks it is childish and stupid but for a variety of emotional reasons he could not break the addictive nature of his behavior and he went into therapy. It is kind of interesting that his addiction to porn had nothing to do with sex, really.
If your husband has this addiction he may have difficulty not protecting it. Progress really began to happen for myself and my husband when I let him know that I was not denying him access to porn, but was very honest with my views about it. I was also honest with the fact that I find it difficult to respect that particular behavior. That is my point of view and I am entitled to it. I don't have to like porn, and he doesn't have to like that I don't like it, but neither person can conform to another's view point unless they are truly swayed in their own heart and mind. If he had wanted to keep porn it would have been a problem we would have to revisit often in our marriage, but it wouldn't end our marriage. I guess I'm saying the problem wasn't so much the porn as it was the dishonesty. I can handle just about anything, except a lie. If I don't know what the truth is, I can't deal with it.
He owes it to you to let you make up your own mind about his behavior. This may be way too threatening if he really respects and loves you - his fear of losing you or your respect might be too much. Though it can also be a lack of respect/selfishness issue - and I can't know in your case which it is, only you can.
Stand up for yourself, but also give your partner the room to stand up for himself. This is such a touchy area for so many marriages and yet often times people just kind of ignore it.
Face it head on, in the light of day you can deal with anything.
I hope this wasn't too confusing - I'm emotional about this so it can be hard to write about.
I am thinking of you and your husband and sending strong, compassionate vibes your way.