Some say that non-monogamous relationships are natural. They believe that humans are not biologically inclined to mate for life. I would say that for every person who had an affair there's probably a reason to justify, in their eyes. My opinion is that we set our expectations of what marriage is too high. When expectations are not met, we look outside of marriage to someone who can meet.
In general, the company looks at the other man/other woman as being responsible for a deal. It is understandable that the other man/other woman become the target for the anger and the anger you feel the spouse cheated. Blaming the other man/other woman would have to take responsibility for marriage problems and our feelings, so we like to pretend that if it weren't for that other person there would never have an affair. Problem is, there was, just would have been another man/woman in the other.
If it is to react in some way. The big question is how to respond. The answer, hopefully in a way that does not exacerbate an already difficult situation. You may feel the need to spy on your spouse if you are suspicious of their activities. This is understandable, especially if you are considering a divorce and need to protect your financial interest in marriage.