Why do people have extra-marital affairs?
Some say that monogamous relationships are not natural. They believe human beings are not biologically inclined to mate for life. I say that for every person who has had an affair there is probably a reason to justify it, in their eyes. My opinion is that we set our expectations of what marriage is too high. When our expectations aren't met we look outside the marriage for someone who can meet them.
Who is the other man/other woman?
In general, society looks at the other man/other woman as being the responsible party in an affair. It's understandable that the other man/other woman become the target for the rage and anger the deceived spouse feels. Blaming the other man/other woman keeps us from having to take responsibility for the problems in the marriage and our own feelings, so we like to pretend that if it hadn't been for that other person there would have never been an affair. Problem is, there would have been, it just would have been a different other man/other woman.
How do you know if your spouse is having an affair?
Determining whether or not your spouse is cheating means distinquishing between the "feeling" that something is wrong and the reality of the situation. You feel in your gut that something is up due to a change in behaviors on your spouse's part but when you attempt to discuss your suspisions your spouse shuts you down. Your spouse has become distant, he/she is working longer hours or, your sex life has died off. All the signs are there, I encourage you to pay attention to the signs but don't confuse signs with proof. Don't make accusations you can't back-up with proof!
How should you respond to an affair?
If it comes up, you are going to have to respond in some way. The big question is how will you respond. The answer, hopefully in a way that doesn't make an already bad situation worse. 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 divorce and need to protect your financial interest in the marriage.