Sorry you are going through this...both of you that is. I am also monogamous and believe I can fully appreciate what you are feeling. BUT...I have also been a cheater in the past so I can speak with some experience about your husbands behavior. He got busted...plain and simple, you caught him and now it's desperation time. This is a very huge hurdle to get over when trying to engage in a mono poly relationship after trust has been broken.
Here's the big question - why do neither of you want to end the marraige? Is it because of finances, social circles, family embarrassment, children....or is it because you genuinely love him and he genuinely loves you? I don't mean love as in the room mate sense (I did that for several years hence my affair). I mean in the romantic partner sense.
Is your husband really polyamorous or does he just want sexual diversity and adventure? People who know about being poly and behaving in an ethical non-monogamous way are held to a higher standard than those who simply cheat and have affairs. Poly is potentially less damaging than affairs but it is also almost guaranteed to be way more work. And that is the main appeal of affairs...less work, more fun until someone finds out.
Do not let him put any of this on you. Work with him if the relationship is worth it but protect your health. Be loving but don't be manipulated or convinced that your monogmay is wrong. Get counselling for both of you and seek out support for yourself - not just here, but real people to talk to.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over