I sit with many couples that have gone through adultery, or in the modern vernacular, "having an affair". The terms don't exactly mean the same thing. One of the definitions of affair in the dictionary is "a sexual relationship between two people not married to each other." Adultery on the other hand is defined as "voluntary sexual relations between a married person and somebody other than his or her spouse." Technically, having an affair does not necessarily carry the weight of betrayal associated with adultery. And betrayal is a heavy weight to carry.
Another term for such behavior is infidelity. I think this definition captures the event in more descriptive and accurate language: "unfaithfulness or disloyalty, especially to a sexual partner." Interestingly, infidelity is also defined as disbelief or "the absence of religious belief"; thus the term "infidel" or nonbeliever. The authors of the Old Testament would often describe the Israelites who turned away from God toward pagan gods as adulterers. They were disloyal and unfaithful to the covenant they had made with Him. He was to be there God and they were to have no other god before Him. A marriage is a similar covenant between two people. We are to be faithful to one another, and not just sexually.
Problems with faithfulness begin way before sexual unfaithfulness occurs. Infidelity may happen often and in many ways other than committing adultery and we need to guard our faithfulness and loyalty to our mate. When we are unfaithful or disloyal to our mate we betray them, just like we betray our Lord when we are unfaithful to him. Heavy stuff if you're not an infidel!
What does it mean to be faithful to your mate? One place to begin is to pay attention to how you think about, view, and treat your mate. How faithful are you to one another in the little things? Do you respect what they desire even if it seems silly to you? Do you defend one another and stand up for each other in the face of criticism or attack? Does your mate know that you are there for them no matter what, or do they wonder if they have to stand on their own? Can your mate trust your word; your promise to do what you say you will do? If you ask your mate: "Do you believe, know that I am on your side; that I stand with you through thick and thin (or sickness and health) and will never leave you", how do you think they will answer?