Infidelity is an unskillful coping mechanism. Any other story is built on the lie of wounded egos.
Regardless of how you experience the intention and impact of infidelity, we all are manipulated by the stories, expectations, promises, ideals, traumas, and fears we bring to the experience. We are the fictional Everyman seeking to protect the imagined safety of the stories, even as the stories eat up our lives.
I say this knowing just as Painter or Beatrice are not responsible for my betrayal, lies, and secrets, I am not responsible for theirs. At the end of the day how each of us reacts or responds to others is about our relationship to our emotional selves. Essentially, the ways in which we rigidly grasp our stories, imaginings, and entitlements define how we react or respond to the world around us. It defines everything from our suffering to our joys.