Earlier quoted context omitted.
Do you have any links to evidence for this being the case? Honestly curious.
Evidence? Sure, I know many couples. The bit about differences between couples who have only slept with each other is my own theory based on my observations. But it seems this has been studied more rigorously, for example: https://onlinelibrary.wiley.com/doi/full/10.1111/j.1741-3737... But this talks about marital dissolution, not simply affairs. Affairs are happening way more than you think.
But of course that's just people for whom I have the data, very small sample.