Hugs to you - that feeling must be a little bit like dying, and my heart goes out to you. There is nothing easy about facing a poly situation when a) it's not something that you want and b) isn't even necessarily something that you knew existed.
Use this board to tell your story, share your feeling and journal. It's a safe place with tons of people who have been through similar challenges; maybe not identical to yours, but at the very least, some enough to provide you with compassion and sage words of wisdom during a difficult time.
I think the thing that you're realizing is that this isn't going away. Your husband has this idea in his head/heart/soul, and it fits for him. Now it's up to you how YOU want to proceed. If it is something that you don't want, that's totally okay - you may have to make the impossible choice of walking away from your marriage in order to honour the ideals that you hold around relationships. That is your right. As fucked up as this may sound, it's your husband's right to express, and ultimately get what he wants out of life as well - not against your will - that would make him an asshole (in my opinion, anyhow - maybe not in everyone else's).
You have SO many rights if you do decide to stay with him and open up your relationship with him. It is not an all or nothing thing, I don't think. You have the right to make agreements/boundaries/rules with him, you have the right to set limitations about how much & what kind of time he can spend with other women. I think it's all about perspective. If you come at it with the idea that you might lose him in the process of poly, everything about poly is going to seem like a threat. If you look at it as though it's ALMOST like a hobby - like bowling, with benefits - you might be able to start seeing it as an expansion of a particular part of himself.
Questions for you - why won't he have BDSM sex with you? Can you see yourself being poly with anyone else?
Share, process, and remember you're amongst friends here, even if some people are WAY more okay with things that you're not (yet, or maybe ever).