I don't subscribe to the view that "once a cheater, always a cheater" either, provided the cheater learns that this is actually a rubbish way of resolving difficulties and expressing dissatisfaction in a relationship. We can perhaps all cite examples of secondary relationships that have been successful having started as an adulterous relationship, but the protagonists in the one I know about agree that they have taken those lessons learned into their new relationships - and feel very sorry and regretful that they treated their former partners so shabbily and deceitfully.
I'm afraid I know far more people who went on to have second and third relationships like this, never having learned that lesson. So when the "new" relationship has perfectly normal strains and stresses, their reflex response has been to look outside the marriage rather than attempt to resolve issues in a more adult way.
Brahms asks why, if it is so wrong, so many people engage in "overlap relationships" like this. I think this is to do with a complex mix of factors: a dislike of being on one's own, a lack of personal responsibility, an unwillingness to engage in true intimacy {it is much easier to be superficial with a succession of new people than with someone who knows you well) plus a whole host of other personality related factors, such as selfishness, narcissism and a need to control people.
So many people claim the relationship - or the person - they left was flawed and so this justified deceiving them. This fails to take responsibility for trying to resolve those issues in an adult way and not choosing deceit instead. If the deceiver is subsequently unable to look back and at least acknowledge that wrong - and look inwards at their way of dealing with issues - they will never learn and will go on repeating the same mistakes over and over again.
When ever I see people in the public eye doing this, it always seems to me that it stretches credibility that every relationship they ever had was lacking in some way, or that they just had bad luck choosing the wrong partners. The fault probably isn't with their partners, or even their relationships. It seems more logical that the fault is with them.
Marriages break down for all sorts of reasons and the pain is horrendous for the parties and the children involved, but what so many posters on here overlook is that it is always worse, for the betrayed partner and their children, if infidelity is involved. Children can rationalise that their parents fell out of love, but they find it much harder if they believe it was because of a third party - and it can feel as though the leaving parent has chosen that person over them as well as their Mum or Dad.
I find it really annoying when posters on here say that infidelity "just happens" and "that's life" - as though this is an involuntary act that people cannot exercise choice in.
It really comes down to whether you believe that infidelity and deceiving others is wrong. I understand why it happens, I think it's behaviour capable of change - but it doesn't get away from the fact that it's wrong. If people however believe that it is "right in some circumstances", they will always be vulnerable to it as a behaviour choice.
Similarly, if posters truly believe that the third parties [OW and OM) are blameless and bear no responsibility, this yet again ignores individuals' social responsibilities. I wouldn't feel comfortable knowingly deceiving a stranger, to whom I owed nothing and had no relationship with. If I thought that deceit was also going to cause that stranger and their children pain, I could not collude in that. Quite apart from my own self-respect and esteem, I would believe that behaviour to be wrong. If I went ahead and did that, I would expect to be blamed. I would feel guilty - and as far removed from smirking at the person I'd deceived as it is possible to be.