What do I know, but I agree with oldtimer and Puppy.
If you want to save your M, I think you should go to your wife and compassionately tell her the conditions to do so. MC for the two of you, and maybe IC for her to understand why she did what she did. Tell her truth about the woman on FB. On the flipside though, tell her you want to set the past aside once in a while and just go have fun with her. If you can't go and have fun with someone, why be with them? If you're not interested in having fun with her, well then there's not much more to say.
If you can't get over what happened, and you can't ever trust her again, tell her that, and push forward with the divorce.
