Hmmmm, so Matt was maybe angling for that "comfort zone" that he was already used to, rather than risk believing that monogamy might be here to stay? but would he have left you as soon as you took up on that "offer?" Yikes, I guess when I compile all the data I have so far, it does seem possible. I don't blame you for not wanting to mess with a good thing.
Hope nothing I said caused the other good thing to be jettisoned; I know this best friend of yours is a good lady and far more trustworthy than Si. But I guess on the other hand, if I effectively voted for your marriage with Matt to continue, then I believe I voted right, and I'm glad that you were already of the same mind.
From my dubious atheist viewpoint, I always fancy that given millions of years to live, we'd all be able to explore a lot more relationships without so big of a risk. But even if there is an afterlife, we still all have to decide what we're going to do with the precious mortal years that we have. Your post and decision show a profound love for your husband. I can't be sad about that, I think you should follow what your heart and mind are already telling you.
Love means never having to say, "Put down that meat cleaver!"