Decades ago I was (one of) the third parties involved with one of a married couple of cheaters. Their reactions were not anger and indifference as in the case of Elle Silver and her ex-husband. In the case I was involved in, when they mutually confessed, the husband was thunder-struck and weeping and regretful and then some weird morally superior "this nonsense has to stop" bossy. The wife was unsurprised and calculatingly weepy and apologetic and promissory.
The wife and I were colleagues, we shared an office. The husband (of three months) worked and lived in another city halfway across the country. I was engaged to my girlfriend of 4 years who was in grad school all the way across the country. The joys of academia.
The wife and I manoeuvered (how do you spell that?) each other quite willingly into a situation to maximize the likelihood of us getting involved. While it was meant to be purely sexual (and collaborating on our research) we were both surprised to find ourselves intensely romantically involved as well.
Two days later after the affair started I was at the university the husband was at, to give a talk, and guess where I was putting up. Enjoyable discussion about our work, philosophy, travel etc. ensued. I was even witness to a very loving husband-wife phone conversation. After dinner, the husband mentioned that he was going to India for a conference, and planning to travel for a few days after. We talked about places he might want to visit, amongst them a wildlife sanctuary he was particularly keen on, where he had a biologist friend who'd invited him and did I know some way of getting in touch with her. This was in the era of uncertain landlines and people posting letters to each other, even to hook-up. He seemed awfully eager, in a more than "oh if she is there great else I'll have an adventure by myself" way.
I mentioned this very casually to my officemate, who just as casually said that her husband probably wanted to sleep with the friend in India who'd made no secret of her availability to him. The wife seemed quite blasé about it and confessed that she really didn't care; she'd stopped being jealous when they'd gotten engaged, and no, they didn't have an open marriage. What did I know, I was just an innocent Indian boy off the boat, so I went with the flow!
My girlfriend and I broke up when I told her about my affair. She was shattered, but months later we agreed we'd dodged a bullet, though I'd been a complete jerk in how I treated her and brought it about.
Meanwhile, my intense affair with the wife continued for a few weeks. Then I had to go back to hubby's city for another talk. He was back from India, but hadn't visited his wife yet. We met for coffee, over which he shared his pictures from India. Crowded cities, overloaded buses, smiling people, elephants, all went well until he came to a picture of him with his Indian biologist, at which point his hand shook so much the coffee cup rattled and spilled on the table! I knew. This was the second time I'd seen that happen, and not the last.
Soon after, I returned to my place of work, while the wife went to visit her husband. She called me, in the midst of mega drama on her end, to tell me that he'd confessed his affair to her after they had sex, that it was a mistake, then she felt obliged to confess back, which came to him as a shock and left him shattered etc. etc. so they exchanged words of love and regret and never again and how much they meant to each other, and the wife composed a letter of apology for me to send to her husband so he could be at peace which would ensure that the wife and I could keep seeing each other, even after I moved away. Three or four more cycles of this until I told her not to call me anymore. Every once in a while, when I am single, I'll fantasize about the sex we had, other than that, I've never even been tempted to look her up on social media.
My decision when my head cleared was similar to Elle Silver's now: to be cognizant of the signs of ill-health in a relationship, fix it or break it as healthily as possible and to never let it get to the point of fulfilling my unmet needs in an affair. There is an internal mindset button that controls an “availability signal”, and I just turn it off when I am in a relationship.