I think part of the problem is that a person who has not witnessed an MLC happening has no idea what it's like.
Our marriage was progressing along, not perfectly, but with a lot of love and happiness, and then, BAM, a tree fell on the house. But it wasn't an affair, it was the news that my MIL was on life support, now brain dead, and needed all the family there to agree to let her go.
My H flew to the country where she was, she was unplugged, and ... my H changed instantaneously. He was unable to make up his mind to come home until his father reminded him he had a family to get back to. He returned, but not the same loving, playful father. He ignored or yelled at the kids, and complained he'd be too old to enjoy himself when they left home. He started making any excuse to be away on business trips. He'd go to bed early and lay there staring with dead eyes. He refused to talk to me about anything, and if I got upset he'd do passive-aggressive tactics that made me look bad. Soon EVERYTHING made him angry (and nothing was ever his fault). He started drinking more. He declared he'd always wanted sportscars, motorcycles, hip clothes, etc etc.
It was 2 years after his M died that he met the OW, and although she then became the focus of all his attention, she wasn't the real problem to working on the marriage. His MLC depression needed to be addressed before he could even care that he had a marriage.
I truly believe there is a difference between such an MLC affair and the exit affairs, etc, that Puppy listed. But until you've seen someone literally change before your eyes like that, it would be impossible to credit.