Marriage infidelity is often the biggest reason that marriages end. Truth is, there have probably been problems in the marriage long before an affair occurred, but often the affair can turn out to be the breaking point. The key then is to deal with problems within the marriage as they arise and not leave them to the point where an affair happens.
To avoid marriage infidelity make sure that you and your spouse deal with any situation that is causing ructions within the marriage. The worst possible thing that many couples do is to ignore what is staring them in the face. Hiding from your problems will only make them fester and grow, get in there and tackle what is wrong head on!
Because the truth is that the affair is usually a symptom of marital problems. If there was emotional distance between you and your spouse, then within that distance there is the chance for an affair to begin. Sit down with your spouse and discuss how you feel. If you genuinely feel that your marriage is not how it should be then you have to be upfront and honest.
If you believe that you need to spend more time with your spouse then say so. Schedules need to be looked at and plans need to be made for when the two of you can spend some quality time together. Spending time together is a classic way to defeat distance in a relationship.
Another classic reason for marriage infidelity is money! Financial problems are without a doubt common reasons that people look for what they need outside of marriage. If you are struggling under the weight of trying to make ends meet financially, of coping with debt, or you're worried about your job security, then again you have to talk to your spouse. Trying to cope with this on your own will only make things more difficult. Looking outside your marriage for comfort from a third person will only hurt your marriage.
If you've got to a point where marriage infidelity is already a reality and you want to save your marriage, then you will then have to put out some fires and put them up pretty quickly. First of all end the relationship with the third person. Make sure that your spouse knows that you have ended it. Apologize for your mistake and explain to your spouse why you think you made the mistake. Any reasonable person can, IN TIME, forgive. What will erode forgiveness over time is lie after lie and betrayal after betrayal. So clearly and honestly explain your mistake, detail to your spouse what you are doing to prevent making that mistake again and live up to your promise!