Suave

Ahh the helpful grammar Nazis. This is probably bad advice, but I'm going to give it anyway because it's what I would do in your situation (as far as I know). Bring up the fact that she fucked your friend in the first threesome, which I'm assuming she did, and that combined with her saying "yes", you were not to blame for the incident. Not only did she allow you to do so, she also set precedent for it.

Looks to me like she had a change of heart during your little escapade, and quite frankly, there was nothing you could do to prevent it. People do that (women more than men), and it seems you'll just have to deal with it. I already stated the way I, at this exact instant, would deal with it, but it's probably not the best for your marriage (or maybe it is). Women are crazy, and you should know that by now.