Sometimes the stage is set in Christian circles for an affair because we try to live too narrow a life. Narrow may not be the best word. Safe. We try to live a life too safe and predictable. Our souls are made for more and at times that may emerge in unhealthy ways when that 'more' is suppressed.
I have a friend who says there is nothing straight about the narrow. It's like a winding two lane highway through the countryside versus the wider more predictable interstate. Sometimes it's good to take the interstate for speed, but if that is all we ever do our souls will atrophy without the ups and downs and twists and turns of the more unpredictable two lane that may or may not have wide shoulders.
We are made for more. Sometimes that is misread and an affair occurs. That never excuses an affair. There are an infinite number of better ways to address it and stretch our souls and have life in our marriages. But in hindsight, in the wreck that occurred in a predictable life, we find a soul that atrophied and suddenly was awakened by one who was not his or her spouse.
Being aware and feeling the impact of both sorrow and joy is a way to be on the narrow, winding road. Owning what is wonderful and what is painful often reveals God in ways we didn't imagine. Owning that and sharing it with one's attentive spouse is a surefire way to build closeness in marriage.
Here is Nouwen,