Would he kill himself to get you back and prove to you that you come first, before his liking for porn? Because if he wouldn’t, then you are NOT the top of his list and should not marry him. Any woman should be absolutely certain she is the most important thing in the world to the man she is marrying (unless he has children already, then they should be first).
Basilisk, respectfully, I took the “would he kill himself?” part almost literally the first time I read it. I know it’s hyperbole, but I believe it goes too far. I would partly agree with Paulin that an ultimatum is not a great thing. It may be justified, but it may still bring resentment. Men don’t take well to ultimatums. If a woman told me, “Either you stop talking to Insert_Female_Friend_Name or I leave,” I’d reply, “Au revoir, mademoiselle.” Now, porn is all the much more and if I did something very wrong and of a similar kind, I’d feel a lot of guilt for it and my attitude would change accordingly. But I’m not sure how I’d take it and if it wouldn’t kill the relationship. While porn is wrong and the need to stop looking at it is obvious, an ultimatum is an act of force, an exercise of authority. It may lead to submission out of fear rather than a normal choice with normal results. I am not saying it would be a wrong thing to do in the OP’s situation. Perhaps it might be a good thing, but only if it’s a simple matter of choice, “choose them or me, you don’t have much time,” not if it entails a desire to control the man’s behaviour, make him give up his bad habits for the woman, prove he puts the woman first and so on and so forth.
A statement like, “prove me you love me,” would make me want to reply, “very well, I give you back your freedom.” Now we both know I’m mean and I’m a jerk, but I probably have some merit here. There are things we don’t take well in relationships and we aren’t obliged to marry someone who does those things. Some of those things are plain wrong and some also constitute an act of betrayal of some sort, although the actor may not realise this (I suppose many modern porn-lookers don’t think they’re betraying any commitments, as wrong as they are). We are entitled to make our decision to leave. We are entitled to ask for change, sure. But we aren’t entitled to control our chosen one’s behaviour. And as I write this, I realise I’ve been guilty of it in the past. Bringing it up is fine, asking it to stop is fine, an ultimatum might be, but it simply shouldn’t cross the line of blackmail and control.
Next, even though, “you need to make your choice,” is fine, perhaps it would be better to explain why it is wrong and why it hurts you and how it makes you feel, then honestly admit you can’t accept it any more and ask the other one for decision. If we want that person to stay with us rather than choose the evil over us, we should be ready to offer some help rather than considering it the other person’s own responsibility and a test.
And here, I would elaborate a bit on what you wrote later in your post, i.e. that the woman should be the most important thing in the man’s life. I don’t agree fully (a man leaves his mother and father by marriage, not by betrothal), but at any rate, it works both ways. If the man is to put the woman ahead of himself, so is generally she to put him ahead of herself. That doesn’t mean accepting his looking at porn, but it does mean that while putting her foot is okay, even leaving is okay (whereby the relationship ends), threatening him with leaving (which I don’t say your position amounts to) is not and neither is deciding that he’s filthy, subhuman, unworthy of you until he gets rid of the problem on his own, that kind of thing - and still continuing the relationship.

Either we want to be in a relationship or not and effecting a change by fear, or making a compromise and settling with a partner we think worse than we could get, is not a recipe for a happy marriage.
Again, this shouldn’t be read as any form of excuse for wrongdoing or obligation to get along with wrong acts.