Just because she has not told her friends that she is having sex with him does not mean that she is telling them the truth. Although she admits to having a secret friendship with this other man (OM), admitting to having sex with him would put her in the bad light of being a cheater, so it is doubtful that she would admit this to them. The truth is since she already has had sex with him in the past, and admits to wanting to have sex with him now, it is highly likely that she has already had sex with him. While your are at work paying the bills, she picks him up in your car to spend the day with him for gosh sake.All her friends (who I liked) knew about this affair. I know she hasen't had sex with him because she told her friends that she's not having sex with anyone random but, if she decided that, it would be with him.
She was surprised that after you caught her cheating that the issue was not that you were going to end the marraige, but how much it hurt you and how much you wanted to save the marraige. She lost what respect she had for you right there. Now she is in full on disrespect mode knowing that she can have you back anytime that she wants. You need to file for divorce and mean it. You can always change your mind, if she makes the effort to earn a second chance.