Yikes, a condom package that mysteriously appeared in his wallet? Combined with everything else, it certainly seems that if he's not cheating on you already (and he may well have) that he will end up doing so soon if things continue this way. That's not what poly is supposed to be.
Dan Savage, an excellent advice columnist for sex and relationships, often talks about the "price of admission", or the things we choose to accept to be with our partners even though that particular thing about them wouldn't be our first choice. Your husband has paid your price of admission -- monogamy -- for the last 12 years. He is no longer willing to and has offered a new bargain. The question is, are you willing to pay it? "No" is a perfectly acceptable answer. I'm sure you've made other sacrifices for the marriage over the years, this doesn't have to be one. But if your answer is no, it sure sounds to me like the other options are that he cheats in secret or you two divorce.
All that said, I can tell you that becoming sexually non-exclusive doesn't mean you won't share anything unique or special. I've been dating a married woman for almost two years and I've seen over and over how special and irreplaceable her relationship with her husband is. I could never take his place and not just because I'm a woman and he's a man -- I'm sure that other poly folk on here who are dating married people would tell you the same. Surely your marriage has lots of special things about it aside from sex?
If you do the thinking and soul searching and decide to try to make things work, though, your husband is going to have to commit to a much higher level of honesty and consideration than he's been showing you. He's going to have to agree to take things slow and put your comfort high on his priority list if you give him this concession. If you go into all this with a lot of unresolved issues or resentment between you it'll be a mess both for the two of you and for any poor women who believe your husband when he says his wife is "fine" with it only to find out that it's not true and they've been put in the unwanted position of homewrecker.
has some great articles on jealousy and on poly/mono relationships. Good luck!