I am so sorry you are going through this. What you have explained, tends to be a typical story.
Many people think they can have NSA sex, only to find out differently. The important thing to remember, is that your wife loves you, and did not plan for this to happen.
Sexual chemistry, usually comes from a 'connection' before people hit the sheets. It makes sense that for most people, that connection only grows, if things are going well.
I can say, I am concerned about the boyfriend slapping her in the face. While I myself understand the enjoyment of rough sex, I cannot fathom just doing it, without making sure the other person is ok with it.
Unless he specifically asked her, and she heard him wrong ?
Having typed that, I can also tell you the other important thing to notice, is your wife hasn`t forgotten you. Obviously she has made sure to tell you when things go awry, and you are her confident. She appears to respect your opinion.
So while it may be very tough for you, it sounds like you are handling things ok, so far.
Talking to others, and 'venting' might be just the help you need. Welcome to the forum.