Thank you all for your advice and comments.
Since I posted this, my partner and I have had a conversation, in which I sensed honesty and clarity from him. I did not feel manipulated.
He has told me that he lied about sex with her and he is sorry for it, that it was not how he should have handled it. He has reiterated that our relationship is fundamental, and that harming our relationship is never on the table.
I am reassured by this, and am willing to give what we have a chance. Looking back, I also can see that I have been interpreting everything he has said and done from the lens of "He lied about this; I can't trust him; I presume the worse."
Your advice that I should ask myself why I am opposed to this is very important, and I am not entirely sure that I have thought that out. He may have been manipulating me into this poly relationship, but I may also have been assessing it through my emotionally wounded perspective. Both can be true at the same time.
So I have decided to go with the flow for now, to be open, and above all, to be open and honest to all parties -- including myself. I will trust him and make my decisions from a positive place rather than a negative one.
This is my hope, anyway. Easier said than done, right? But, emotional manipulation or not, this relationship is worth saving. This has been a bad episode for us...we usually are so much in sync that this lack of communication has left us feeling helpless.
But I feel our conversation has allowed for some clearing of the air. It gives us the opportunity to come at this from a better place...together, rather than at odds.
I hope we can work through this.
Thank you all.