I'll take a middle ground and say that I think some affairs happen in marriages that aren't really bad but find themselves at a really bad point. I think my sitch falls in that category.
I was trying to start a new consulting business without having much success, we were living with W's parents as a result (which my W hated), I was somewhat depressed because I wasn't making money and withdrew from her because I felt guilty about not being a better provider, and she got a new job that made her feel independent for the first time in our M.
There were serious issues in our marriage to be sure, but the biggest reasons that the A happened, in my opinion, were due to temporary circumstances. I suspect that other sitches fall into this category as well.
The LORD is near to all who call on him, to all who call on him in truth." (Psalm 145:18)