So, let me understand this, you are saying that it is impossible to reconcile if you don’t set the boundaries early on? Or, is it that without the boundaries you would have more emotional damage?
Definitely not saying the first.
I'm saying that whatever core (the big stuff, not things like "I need a third party to handle the drop-off and pick-ups with the kids" or something . . . more like "I refuse to live in an open marriage," or "I will not have you texting or phoning your OM/OW in front of the children") boundaries one is going to set, they should set them early.
a) it adds clarity to your position, so that your wayward/walkaway spouse is clear on it, and
b) it can help limit (not eliminate) the emotional, financial, legal and even physical health damage.
My advice (and my experience) is definitely coming from a place where's active infidelity, but there are other posters who have had good success with early, strong boundaries even when there wasn't an affair (Coach and Greek come immediately to mind).
Again, Angel is a very strong and special case, in my experience. She seemed to have a very high tolerance for her husband's MLC, and a ton of patience to wait for him to come back around. Most can't do that, and since none of us have any way of knowing ahead of time how long our wayward spouses are going to remain wayward, my default position on my advice is to try to head into the storm with boundaries (legal, financial, emotional, spiritual) firmly drawn at the outset, to the extent possible.
It worked for me, but as Angel pointed out, the other way worked for her, so it's up to each poster to decide which course is best for them, and what their non-negotiable core beliefs are.
