Lolass if you had expressed your views on that other thread as you have here, I wouldn't have reacted as strongly. It also seems as though you have shifted your position somewhat, because you were then conveying scepticism that it was even possible for an affair to happen in a good marriage.
I understand only too well that in the aftermath of an affair discovery, we all draw on our beliefs about why affairs happen and often, those beliefs are reinforced by a lazy counselling intervention, well-meaning friends and sometimes, a guilty spouse.
However, what I've learned is that focusing on the relational causes of infidelity, to the exclusion of individual and/or lifestyle vulnerabilities, is counter-productive and never gets to the heart of why an affair happened.
It is tempting to look at the most obvious causative factors i.e. an evidently distant relationship and conclude that the disastrous combination of this, with the threat of a predatory OW/OM is what caused an affair to happen. Hence it is all too easy to conclude that if a predator hadn't come calling, nothing would have happened.
Now to an extent that last statement is true - for someone to be unfaithful, they've got to have a partner to do it with, but it doesn't get to the heart of the problem because that suggests the fidelity in your relationship is being controlled by a third party, or the absence thereof.
It also doesn't explain why people who are either similarly unhappy (like Christian perhaps was) or more pertinently, are completely dissatisfied in their primary relationship, have in the past and would in the future, turn down the opportunity to be unfaithful.
At the heart of this is the concept of the "good marriage". What is that exactly? We probably all have our own ideals and strive to achieve it. However it would be naive to think that in a long marriage especially, the entire relationship has been characterised by complete unity, regular satisfying sex and limitless romance.
The reality for most people is that it is peaks and troughs. Times when it is hellish, times when it is brilliant and vast tracts of time when it is "good enough". It is just too easy to shine a microscope on a marriage after infidelity and find causative factors within the relationship, because that doesn't explain why infidelity fails to occur in relationships that, like Christian wisely observed, are appreciably worse.
Now maybe the lack of opportunity to be unfaithful explains that in part and I am sceptical when people say they would never be unfaithful, if it turns out they've never had an opportunity. In my view, they are no more infidelity-proofed than the next person, because it simply hasn't been tested.
More pertinent then, to learn from people who have been tested, when their relationship was in a hellish phase, a good phase and when it was just ticking along, but no matter what, they still weren't unfaithful.
And how does it explain infidelity occurring in marriages that were, at that point, going through an especially good, happy period?
Now I think the challenge is to find out what's really caused it and the answers often lie in non-relational vulnerabilities. That is, a person's individual personality, character, beliefs and behaviour; their individual stories.
Then look at their lifestyle; the culture of their workplace and the beliefs that have infiltrated their moral compass. Whether infidelity is tolerated, encouraged or even celebrated and the only derision from others occurs when you get caught, or fall in love.
Whether the person spends long periods away from home, where opportunities are plentiful and physical and emotional distance make infidelity more permissible.
I have always said that affairs evidently happen in unhappy marriages and that a happy marriage is self-evidently going to be a better deterrent to infidelity, than one that has hit the buffers for other reasons.
However, since it is also true that affairs happen in happy marriages and "good enough" marriages, it is a mistake to focus on the relationship alone, because that is only part of the story and if we scrutinised all our marriages, we would find some level of dissatisfaction or disharmony. I doubt there is such a thing as a perfect marriage, all of the time. We could all find "causes of infidelity" if we looked hard enough.
I don't like talking about an OP in the third person, but in my view, it would be a mistake in Christian's case to point to the marital problems and conclude that they were the sole cause of the infidelity that occurred. They certainly didn't help, but creating a revitalised relationship in the wake of disaster is only part of the story, because this buys into the prevention myth, which is that a "happy marriage" is immune to infidelity.
Unless the other causative factors are addressed (individual and lifestyle) there is still the chance that infidelity will blight the relationship. Hence, a more holistic approach to finding cause and effect is by far the most sensible way of moving forward to a happier future.