Choice aside, often the broaching of the subject itself(sex with others) is very destabilizing, even in fantasy to an anxious partner. They now have to live with knowing this is something she wants and wanted enough to bring it up for discussion

So I can give you possibly an unexpected outcome from your suggestions that also provide a look into the way an anxious attached person might see things.

If the nm person suddenly ramps up their time and affection for their primary partner beyond what has been normal for them, it can seem insincere and actually have the opposite of the intended effect.

The anxious person will think, they are only behaving this way toward me so they can sleep with or as a result of sex with their boyfriend/girlfriend. They don't really feel this way, they have not acted like this toward me before.

Remember when you were a kid an engaged in "good behavior" so you could get something you wanted, Say you wanted to do something but your parents said no, so you began cleaning your room without being asked in hopes they would go along with what you wanted. This is what it can feel like if due to a desire to engage in nm or as a result of the nre you treat your partner in an abnormally nice way.

The difficulty with your situation and you are far from alone, is the idea that through study, conversation, reading books, etc. your husband can logically override his feelings about opening your marriage.

The trouble is logic and emotions speak different languages. So nm logically seems like it could work but it does not feel like it will. And even when you get into all the transactional stuff about meeting your partner's needs, etc, in the end it does not feel comfortable.

This is a tough impasse to overcome because the brain is always trying to convince the heart(emotions) that everything is ok when it does not feel that way. It is tough for the nm-wired person to understand this because it is simply not how they feel.

I am saying she has changed her interest in their original marital agreement, as do many mono married people. Divorce is likely the best option rather mono or poly if you no longer wish to be exclusive with your spouse and they do not want an open marriage.

In your original post you did not mention what it was you were hoping to get out of opening your marriage. While I know this can be a tough question - what are you hoping to get from this?

There is nothing wrong with you. You have changed and now want something different than when you entered into a monogamous marriage agreement.

You may have to get divorced to get it, but there is nothing wrong with you

I know this suggestion has not gone over well in the past, but I will put it out again:

Why not do a trial separation for a year? You will get to try NM and see if it is what you thought. He will be free of the "let's open our relationship elephant in the room" and you both will have a fresh look at who you are and if you want to stay together.

That is a cool metaphor! The anxiously attached will struggle with the concept that the relationship is "irreplaceable". But for securely attached it works!