First off, I want to suggest you tone down the enthusiasm with respect to the site. The site's author is self-serving. I did not read in-depth, but I did read enough to grasp that the author is in the business of selling marital help. Apparently, in order to increase the demand, he claims that marital infidelity is the worst thing that can happen in a marriage. Clearly, he has never lived in the real world and has never heard of people being held at gunpoint and/or choked to death by their spouses.

Also, and independently, his ideas are highly simplistic. I bet it helps sell marital help - complexity is a much tougher sell. So on your idea of taking the concept and applying it widely:

When you are trying to take an oversimplified idea that was oversimplified even within the boundaries of the original context in which it was conceived, and, expand it to apply outside of that context, you might be following a wrong path.

Secondly, I personally do not use the concept of forgiveness at all, but I do see the value people bring to my life even if they are wrongdoers. Eventually, even in the context of people who clearly offended me and offended me deeply, I still see some positive and still see some silver lining. Some lesson, some blessing, some thought they provoked that eventually caused good things - along those lines. It has nothing to do with forgiveness, and everything to do with dialectic/duality.