Do you even want to polyship, hon? It is ok to tell him NO if you do not.
You cannot stop him, but you could be honest about how you feel so at least you are in right relationship with your own self and not keeping secrets from your husband. He might not like what he hears, but he deserves the truth. Do not lie to you spouse. Do not lie to you. If you are NOT willing to participate in a polyship SAY SO.
This business of going along with whatever other people want because you don't want to deal in conflict resolution? That's not healthy. Putting it off and putting it off -- it just gets bigger and bigger.
Learn to do healthy conflict resolution
in the "nip it in the bud" stage. This is not the enemy. This is your spouse. It is opportunity to grow and reach new understanding of each other if you want it to be.
If you ARE willing to participate in a polyship but not like THIS at too fast a speed? Speak up. Say so. He deserves the truth then too. So do you.
A healthy polyship without communication and truthiness. Don't be starting out that way when building polyship foundations.
You have to finish the stages of grief even if you willingly choose to open. Because once open, you are not closed. Even if both gladly want to open, you are no longer closed. Maybe this post helps.
You haven't even met the woman. Much less talked in trio about what each player has for their personal wants, needs, and limits and what they hope to get out of the polyship. See if what she is after is even compatible -- dating time is for that. To find those who are compatible. You 3 have to come to agreements for how you want to BE in polyship so you can hold each other accountable to whatever polyship standard you all create for yourselves. Call it the "our code of conduct" if you like that better. Call it fluffypants. Whatever you want to call it... MAKE YOUR AGREEMENTS for how you each want to be treated and how you want to treat each other so you can all be treated WELL inside polyship.
Without talking about how you want to open and how fast, you will feel like you are along for the ride on a runaway train or something. You have not even agreed on what open model relationship
you want to think about practicing with your polyship people.
It's hard to feel balanced when nothing is solid.
Could talk things out first before giving the green light.
I would suggest you take it BACK and tell husband "NO. I made a mistake. I apologize for that. I gave a wide open green light when I was not prepared. I was confused because I felt pressured. I should have said I need more time to think if this is even something that I want to participate in or if I have the skills to do well. I need more information and agreements before agreeing to sign up for this new mission. I want to know what I'm getting into."
Then both of you take a time out to regroup so you can do this WELL if you are going to do it at all. This ASAP.
Right now? It's starting off on the wrong foot. That's no good to anyone.
Because if you cannot even speak up for your own wants, needs and limits to be known? So you tend to 1 person in the care and keeping care of you and preserving your own best healths?
If you cannot work with him in and be 2 people tending of the marriage so it is healthy and the health of both people in there are doing ok?
What makes you all think that you can change it to 3 people in a polyship and do it well as a team? 3 tending to the health of the polyship and the 3 people healths inside there. Regardless of what model you choose to practice?
More people = more wants, needs, and limits to have to balance.
I mean this kindly. You are not horrible people. But you are taking on more than you are prepared to deal with at this time.
Prepare first. THEN if you want to go there, go in there better prepared.
That is all.
Talk to your husband. Speak your truth, if even at a whisper. Sort yourselves out. You can do it.
If the choices you face right now are "give up, give in or give it your all?" Give it your all, hon. It's your life. It's not a dress rehearsal. You only get the one. Pick how you want to be livin' it.