Let's go back now to Cindy, Martin and Frank. Cindy had become very attracted to Frank. He offered a refuge from the pain and distance Cindy was feeling in her marriage. She became focused on what Frank did well that Martin either did not do so well or had not done recently. That's what happens when you think seriously about alternatives: you become focused on the things that the alternative appears to offer, usually failing to think clearly about the things you already have in your mate or that the alternative does not have. Hence, your perception becomes skewed.
While Cindy was thinking that Frank might have more to offer her than Martin, the truth was that Cindy and Frank were on their best behavior with each other. They didn't have to work out budgets, discipline kids, clean up the kitchen together, or do anything hard. They could just listen and talk as friends. But it wasn't real life, and it wasn't a fair comparison. Cindy was actually more compatible with Martin, but she had lost touch with him.
You won't get far battling the temptation of alternatives if you don't recognize and accept your built-in bias. You have to push yourself to doubt your certainty that someone else could be as good or better for you. Even if you can't find anything wrong with that person, you need to recognize the importance of the promise you made before your mate, your friends, probably before your family, and before God. The promise you made is serious stuff.
If you are currently attracted to someone else but you are committed to your marriage, take a hard look at how your perceptions of the other person may be biased. If the attraction is a serious threat, ask a close friend you trust to challenge you to build and maintain your commitment to your spouse.
One note here before we move on: although it is often biased perception that makes an alternative seem more satisfying, it is possible to become attracted to someone who actually would be more satisfying to you than your mate. There may be something missing in your marriage that you desperately want, and although it might develop later, it also might not. It's painful to have this realization. It can also make you resentful and angry. If that's your situation, it's better to acknowledge it and grieve for the loss rather than letting it erode your dedication to your mate. Otherwise, you could lose all you have built together.