Hi S, I'm in a similar situation to yours, not married and not rushing.
Just a couple thoughts. Lets look at this objectively. Can you imagine dating a girl for some time, bring up the subject of marriage and say straight to her face "by the way, I have an addiction to porn (or looking at things I shouldn't) and I do that whenever I'm in an uncomfortable situation". What do you think her reaction would be? "Oh, that's fine with me."? Personally, and I speak for myself, but if she said that, I would be very uncomfortable with agreeing to marry such a girl. Quite the contrary, if she heard that she would go off running. AND, she would go tell all the girls she knows as well.
When you go to the store to buy something, you don't care what manufacturing facilities and what materials were used in producing the object. You're just looking at the finished product and thats what you want. When a girl is looking to marry someone she wants to see the finished product. Why does it matter how you got to where you are?
If you are someone who is not interested in recovery and wants to remain the way you are. Then true, the girl you wish to marry deserves a full disclosure because THAT IS WHO YOU ARE.
However, if you are someone who is in recovery, working the steps, and as SB mentioned before, are working with the LUST addiction, then presumably (I might be wrong here) you won't wish to get married until you see that you are controlling your addiction. If I thought that I can get married in my current situation of still getting my bearings in recovery then I would say that I have to go back and do my First Step again.
Once you are in recovery, have made sufficient progress and have begun living life, then what does it matter to her HOW you got to this stage?
An open marriage (as I understand it) is regarding things which are relevant and affect both of you. Feelings towards each other, actions that you do which affect the other and such things. Obviously if acting out and watching porn are part of your daily lifestyle, then that is something that affects her. Just as much as going to a hooker and cheating on your wife affects her. Would you tell a girl you wish to marry that you were once with a hooker? Porn and masturbating are the same as being with a hooker. Because what matters is not the form of the acting out. What matters is that you turn to lust, in whatever form it is, whenever you find yourself in a situation that you need to medicate out of.
I have a friend who's wife B"H does not have any idea whatsoever as to the nature of his "distraction". All she know is that recently he started attending a "support group for men" and that she sees the results in his day-to-day conduct. Many women don't want to know anything about it. If they wish to find out they would ask.
Most of the time (as far as I know) the discovery is made by the wife after marriage by catching the husband acting out. If you have identified the addiction before marriage you now have the ability to free yourself of the addiction to the point that Bezras Hashem you're wife should never find out.
I would very much like to meet the man who disclosed his addiction to his wife before marriage and she accepted it whole heartedly.
I may be wrong about what I have said and am open to comments.
Keep on trucking!
-Mac