The struggle we undergo to remain faithful to someone we love is little better than infidelity.
~ La Rochefoucauld
An affair is a symptom of a marriage that is not working. There may not be enough intimacy, affection or sex, communication may have broken down or a baby or child may be taking up a lot of time and attention. In all cases, at least one of the partners is not satisfied. Usually it is the person who has the most trouble asserting themselves in the marriage.
The guilt around having an affair, even just the thought of having an affair, often eclipses this reality. Guilt is in fact why needs may have gone unmet in the first place: you feel badly about having them and don't assert yourself until it is too late. That is why your partner is shocked to learn of your infidelity. “I had no idea you weren't happy”, he tells you, “why didn't you say something?”
Infidelity is often attributed to moral depravity in the face of temptation and its antidote to the exercise of self-restraint. But is that what holds a marriage together? Is that love? I like to quote Spinoza on the subject. He defines love first and foremost as joy but “with the accompanying idea of an external cause”.
Fidelity is really all about mutual satisfaction.