Tough place to be.
It sounds like you were both unclear on where the other person stood, and it sounds like you really want an independent partnership with another woman. It's unfortunate that it's come to a place of ultimatums, but if your husband is feeling like he was deceived, and now feels threatened, it makes sense that he would be acting from a place of fear and defensiveness. I'm not saying that how he is responding is "right" but I can see where he would be coming from without necessarily agreeing with him.
It sounds like the only way that he is comfortable with you exploring your bisexuality is within the context of a threesome. This sounds like his ideal, but it doesn't sound like yours. The complication of your growing emotional attachment to his woman is certainly an important consideration as well.
How best do you think that you can answer your husband's concerns right now, given that he is putting your marriage on the line? Do you feel like your bond with this woman is strong enough to survive a break while you get clear in your marriage? What do you think your next steps forward should be? Have either of you done any reading on poly? (Opening Up, The Ethical Slut, etc?)
I hear that you're suffering right now, and for that I'm sorry. The beginning of opening up a relationship can be VERY challenging, and you have my sympathy.