Hey, thank you for your replies, this has really helped me. Its hard when none of your friends are mono and the only person who you feel you can talk to thinks you're completely wrong. Hearing everyone's opinions really helped.
Except for you, GroundedSpirit - because my problem isn't the relationship itself, but the circumstances surrounding it. Thank you for berating me without taking the time to understand my situation.
Her and I sent each other some emails which didn't really go well. It was a bit of back and forth hot headedness and ended up pretty much just making everything worse. Much worse.
However at the same time I've been talking with my husband about everything that has been going on, and we've gotten a lot better at explaining how we feel and really got to the bottom of what really was causing us to both feel the way we did/do.
I ended up sending her another email after his and my last talk apologizing for my tone and telling her a bit more of what I felt. My husband also spoke to her about things I said to him about my feelings, also my remorse in being so hostile to her, and her and I have spoken to each other a bit on the phone - we also have plans to hang out again soon.
I feel confident that things will work out. I am still upset with my husband because of how he handled the situation but at this point that is the only feeling I have about everything. I know now that I can trust her and a lot of my fears were unwarranted, and I understand where they are both coming from a lot better now. I am working on helping them to understand why I reacted to their relationship in the manner I did: number one because this really did progress to this point way too quickly for me to be comfortable, and number two because I found I was being lied to by my husband and lashed out at them both for it, even though she really didn't do anything wrong.
So its not a completely happy ending yet but things are so much better than they were last week, I still have some boundary issues that I need to work through but I can tell that as long as all three of us are on the same page they should not be a large issue. My husband and I agreed that this is a rough stage we're going through and the near future will be a bit trying on both of us -- but we are very eager to work this out now so that this does not ever happen again. I'm actually looking forward to this working out, and I see a lot more of the positives of their relationship than I did before now that I've had the chance to hear her story along with his.
Thanks everyone, again.