Wow, what a discussion. Are we confusing forgiveness with staying maybe?
I see this different, though I respect the other points of view. I could stay and forgive or walk and forgive. In the end I need to forgive regardless, because not forgiving (as I understand it) would require too much energy in continuing to hold onto or resent what happened to me. I think that not forgiving, in the sense that I define it, would interfere with my ability to love the way I want and deserve to love, whether in this relationship or in another.
Forgiveness to me doesn't mean that I roll over and stay. It doesn't mean that she is absolved from work toward maintaining the level of trust she's working to build, nor the work required to ensure it never happens again if I choose to stay. It means that I have come to a place of peace with the fact that it happened, however I find that peace.
Forgetting (such a horrible word in the scheme of forgiveness) is totally different. What happened provided information about how I choose to move forward, and that lesson isn't forgotten. We aren't wired to "forget." All of our experiences shape who we are, and they can't do that if we are intended to forget. Whether I stay in this relationship or move to another, that information guides my behavior toward having different experiences than if it had never happened. If I don't do my own work to get through this as a better person with good lessons learned and instead choose to harbor resentment and anger for what happened to me, the behaviors associated with it will emerge in other relationships just as it would in this one. For me, the anger and resentment are linked to my ability to forgive.
Of course, the glaring difference is that in THIS relationship I'm with THE person that betrayed me so horribly. In order to R I have to approach the relationship from a standpoint of rebuilding from the beginning in a lot of ways, which requires work and a demonstration of changes in behavior from my WS that I wouldn't immediately expect from a new relationship. But I KNOW the goodness my WS is capable of by both our history and the actions she has taken since we decided to R. I love so many things about her that I couldn't imagine finding the same mix of characteristics in another person that would fit me so well.
I agree that there is a cost to forgiveness. Though this cost seems to rely on the perceptions of the BS. We can see this as an affront to our choice. I see it that way, too. I would never have chosen her if I had known she was capable of this in the beginning. We can also choose to stick to the way we viewed infidelity before we experienced it first-hand. I don't choose that. My disdain for this behavior goes back to childhood (and my parents have been faithful and are still together). Looking back I had a very one-dimensional view of it. I am now living in the context of it. I am seeing the issues from life experience that lead to the delusional thoughts and ego-stroking behaviors that result in infidelity. Because of what I know and the understanding I have gained about my personal situation, I choose to view staying as a personal emotional strength I never thought I would have, rather than as succumbing to a situation in which I can't win.
Because I don't choose to view infidelity as I did before I experienced the destruction of it, I can't hold myself to the same ideals I had before. I don't see myself as sacrificing my integrity or my goals in life by staying. I see forgiveness as separate from staying anyway. I have to forgive to move on either way. I don't have to stay. But life is about experiencing change, taking the information from it and incorporating that information into the knowledge already gained. I had no actual knowledge of infidelity before this, so to incorporate it into my perception of what it might be like is counterproductive. I have to incorporate it into the knowledge I have, which includes my love of my partner, what I truly know about her, what I know about myself, what I know about others (yay, SI!), what I know about how she is proving her changes, and what I know about our relationship. The choice to stay incorporates all of that on top of my estimation about what is possible for me, her and the relationship moving forward. I move forward with forgiveness in the relationship with the promise of a better one. In our case, the devastation of the infidelity highlighted some serious barriers to expressing the real love we have and feel/felt for each other. A lot of the work is on her for resolving the issues that led to her brokenness, but it also involves work on how we communicate in our relationship together. With this new vulnerability and openness we are experiencing, why would I leave? How could I not eventually forgive? I will not ever concede that THIS had to happen for us to get there, but it has happened. It is happening. I had to lay all the known information on the table and make some very hard decisions. They aren't decisions I expected to ever make, especially in my relationship with her, yet here they are.
Now that I have first-hand knowledge, would I forgive if it happened again? Yes, I would. It would take a lot of time, but I would forgive. However, I wouldn't stay.