...I don't believe there is anything you can do to prevent an affair. Some books/blogs say there is, but in our case there isn't anything I could have done that would have prevented his affairs, since they started before I met him.
Each affair, and the circumstances surrounding it, are different. There are very few rules that fit every one. In some affairs, I do believe preventative actions taken in the marriage would have likely prevented the infidelity. In other affairs, nothing the betrayed spouse could have done would have made a difference.
I know there is always the risk of casting blame on the marriage (or, worse yet, the betrayed spouse) when I write something like this, but I do believe that some affairs are born out of vulnerabilities that have been left in a marriage. This does not excuse the unfaithful spouse, nor does it blame the betrayed spouse. There are always other choices to be made and the unfaithful spouse is responsible for making the choice of betrayal
When it is something within the marriage that causes one spouse to choose to cheat, doesn't it still come down to there is something broken in the one who cheated? Otherwise wouldn't both partners cheat, since they both are in the same marriage?