For what it's worth, my wife (Taiel on these boards) and I are going thru something similar - there is a thread in the introductions section. Except I am the one who, as BlackUnicorn phrased it, 're-evaluated' and asked to open our marriage. We also have a number of poly friends in various 'models' of relationships.
We started to seriously discuss opening our marriage about ten weeks ago and are still doing so. Like you, Taiel is having a very difficult time understanding why I feel a strong need to do this. Your reactions of shock, sadness, anger, hurt and confusion are completely understandable and normal; you have had the equivilent of an emotional bomb dropped on your head.
The best advice that I can offer (from my limited experience) is:
1) Do not make any hasty decisions -- take your time and TALK with your wife. She needs to be patient and be willing to take as much time as necessary. Her biggest temptation may be to push you faster than you are comfortable with -- I know that I have that impulse; I try to control it with varying degress of success. You BOTH need to be onboard with whatever you decide.
2) READ SAGE'S BLOG: Polyamorous People
-- it'll be of tremendous help. Also, get her ebook. (Sage should really consider giving sales commissions ....
3) Ask your wife to read these boards with you. Discuss what you find here.
4) "Opening Up" is a very good book to read. Other books to consider are: "Open" by Jenny Block and "Love Unlimited: The Joys and Challenges of Open Relationships" by Leonie Linssen and Stephan Wik. Both are available on Amazon. I have read the first two and have ordered "Love Unlimited..." (I received a strong recommendation from a poly friend).
5) Search for (and read) posts by "MonoVCPHG". He's a monogamous guy with a poly partner and he gives very good advice.
6) Anita Wagner has a very good guide on how to successfully open a monogamous relationship here
7) Yahoo groups has a group dedicated to support of monogamous people in committed relationships with poly partners. It can be found here
Advice for your wife:
1) Be prepared to reassure your husband that it's nothing that he's done (or not done). It isn't that you don't love him any more. (Or any less). You aren't trying to replace him and in fact he can't be replaced. And I don't mean once. Continually.
2) Be prepared to regularly "date" your husband again. Come to think of it, you should regularly "date" your spouse regardless of whether he's willing to consider poly/nonmonogamy or not.
3) Be prepared to be fully open and communicate with your husband on a level that you've rarely (if ever) done before. I can attest to this -- whether Taiel and I ever reach the point of actually opening our marriage, our relationship is actually much stronger that it was before. We are closer and have talked more in the last ten weeks than we have in years. This is a GOOD THING (tm).
4) Understand that as hard as it was for you to even tell your husband of your desire to open your marriage, it's going to be even harder for him to process. Be understanding, be sympathetic and BE PATIENT. By far, the most difficult part will be being patient and not pushing. You have essentially asked your husband to throw out a lifetime's worth of 'training', turn his world upside down and then put the pieces back together. This is going to take some time. Reread this point ten more times; I'll wait until you're done.
5) Yahoo groups has a group dedicated to poly people with monogamous partners here
I wish the both of you the very best.