I unfortunately can't give the answer because I'm not a WW and I didn't walk in those shoes. However, I can tell you my thoughts on the matter based on my own experience with my exwh.
My exwh moved out in the fall of 2011 under the guise of the ILYBINILWY theory. I suspected and asked him 1,000 times if there was someone else - he always denied and I wasn't able to confirm the A until a few months later. Before then, when he was out of the house and I still didn't know who was waiting in the wings for him, he said he didn't want to file for D. He said he didn't want things to be that permanent. Now, mind you, we are both lawyers and either one of us could have filed the papers without a problem. I even offered to do it and to keep it just between us. He said no. He kept me hanging even though he was spending all of his time with the slunt and planning a big exotic vacation with her.
When I figured it all out and told him he was dead to me, he still never made a move toward D. At that point, I made the decision to hire a lawyer to represent me and filed in early March after all the financials were collected from my end. He chose to represent himself.
He was his own attorney and he continued to drag it out. He wouldn't respond to my attorney's proposals. He would come to court late - once to the point of being reprimanded by the judge - he would lie and say he responded to my lawyer's email messages when it was clear he never did. It was ludicrous. If he wanted out so badly he could have gotten this done and over with lickity split. We never even really fought over anything - it was just his laziness or, what I believe was his extraordinary inability to really want to see what was happening.
I think in these cases, the WS is like a lost puppy. While they are diddling around with these OW and living a fantasy life, they have no idea of the serious consequences that are coming. Once they are hit with the reality that they just blew up their entire world and that of their spouse and children, they shut down.
I can tell you that my exwh is about the most conflict avoidant person you will ever meet. Nowhere was that more evident in the way that he chose to end our marriage. He ran away without ever talking to me. I got one conversation when he was able to muster up "not in love with you" phrase, but that was all false foggy bullshit anyway. He never told me about her, he never told me he was in love with her, he never said a word about her. He just shut his eyes and hid under the bed to avoid the boogey man, just like a kid. So, it stands to reason that people like this don't possess the ability to stand up and face the bad or ugly parts of life. They will muddle through it like they do with everything else.