One of the concepts that I had to take to heart to really understand polyamory is the concept of compersion or frubble. It is that nonsexual joy you feel for your partner's happiness.
Hmm. Never heard it called "frubble". Heh.
Anyway, YES!, that's very real stuff, and can, I think, help Karen deal with her situation--if she can get to compersion from where she's at at the moment.
Karen, It looks to me that you and Matt have a very solid relationship and that there's a lot of good love there as well as good communication on both sides. So, basically, you're just hitting a major and unexpected bump in the road.
It perhaps should not have come as a huge shock to you that your husband would or could fall in love with a woman with whom he's been having a hot sexual relationship. Sex is inherently and intrinsically a bonding excercise, especially when repeated--and anyone who thinks that good sex doesn't always
involve the whole person, including and especially their emotions, needs to take three steps back and look again. What is more, you should thank your lucky stars that this is so about sex, because it is due to this fact about sex that you can feel so deliciously close with -- even "melt" or merge -- with, one you love in this very special way.
The fact that your relationship was already sexually open while not being emotionally open (in terms of numbers of partners allowed to emotionally merge) makes this a very interesting case--and spiritual/psychological challenge. If I were you -- and clearly I am not! -- I would thank the Mystery for handing me this painful growth opportunity. That's right, I'd recogize that the situation feels like lemons and my job now is to make some lemonaide. (The term "Mystery" isn't meant here as a stand-in for "God", but that's another story. Just know that I imagine no sky-Dude in a white beard.)
In my 12 year poly relationship with my partner, Kevin, we've only been fine-tuning our already polyamorous approach to things. Because neither of us has yet had a lasting other partner, though we've both had others for a bit, we've only really flirted with polyamory a few times--other than to live and to be basically open to it and to subscribe to that approach to relationship. I can tell you this: Whenever he or I have been involved with another it has always had the net effect of improving our love and our relationship with one another. Part of this is because being challenged to look closely at what has traditionally been called "the soul" (our 'souls') has always been nourishing of that 'soul'. The 'soul' wants attention--needs it to grow and express itself. The result is an ever more soulful life.
When we run a marathon and win, or come in second..., we can feel proud of ourself, or when we climb a mountain, etc.... We discover our strength by testing it. We grow our spirtual muscles by using them. These lemons are such an opportunity for you. You have a soul mountain to climb. Whether at the other end of your journey you come to saying a heartfelt "yes!" to your husband's desire to have this other person as a true love also, or not, is not what is ultimately at stake in your climbing of your soul mountain. This other relationship isn't at the heart of your climb, rather what is at the heart of your climb is -- as it always is for all of us -- your relationship with yourself. Not your husband. That is, your (and anyone's) primary relationship, which is the basis and the heart of how it is all handled from your end, is the relationship you have with yourself. What this crate of lemons is doing in your kitchen is saying "Hey! Here's an opportunity to look deeper at your soul than you've done in a long time, if ever." There's no possible evasive action, really. That's why the pain is so powerful. It is demanding! It's saying, what? why?! who? when? where?! The pain is a question and you are the answer. That is, only you will know how to dance this dance, one moment and day at a time. But the key is to turn toward and into the pain. Stare it in the face with as much courage as you can bring; never turn away from it! Go right into it with courage and strength -- and I promise you that you will be astounded by your own courage and strength and love. What is more, there is -- I am certain -- great joy to come, however things go "out there" in the pattern of relations. The real core of the matter for you is your own heart/soul.