When you've asked your W for password to FB, etc., have you gave yours to her? You see, if she has nothing to hide...then there would be no reason for her to withhold a password.
Yes, I've done exactly that. I actually wrote all my stuff down and handed it to her and walked away. She still is "considering" giving me hers. She says that that by me wanting those passwords, I'm being controlling, acting like her father, snooping on her, etc. She doesn't like that. However, I think she does understand how it can be important to building trust, so she's heavily considering it. We have a MC session today. I'm going to bring it up then, because I know this MC is a DB fan. This MC has quoted several DB saying on her blog. I know she'll understand transparency as the ONLY way to rebuild trust.
Look, John: transparency (and, more importantly -- the no-contact that it enforces) is either a dealbreaker or it's not. It's either a Boundary of Personal Integrity for you or it isn't.
If it's not, then DROP IT -- you're just pissing her off.
If it IS, then every day that you allow to go by without her being transparent with you, you are giving your tacit approval to her secretive behavior. And quite possibly to her continued infidelity.
You need to decide which it's going to be, and I mean TODAY. Personally, I would bring it up in MCing, and if she refused, then stand up, politely thank the MC for their time, but say "If my wife is not going to be honest with me -- and with you, in these sessions -- then I don't see any reason to waste my time here," and I would leave.