We should be careful about being judgmental about someone by listening to a small snippet about the person's life. Here we have heard only one side of the story. Whether the flirting was serious or not depends on the intention of the guy. Sometimes, humorous flirting can become serious; but in many workplace situations it is usually harmless. We should be careful about advising this woman not to marry her fiancée as "he won't change". Marriage is about trust, respect, understanding, love and compromises. Sometimes a guy may think that it is okay to flirt while he is single; but he may realize that he has to change his habits when he is married. If your fiancée has other good qualities that attract you to him, please do not allow this incident of flirting stand in your way. But, as others have advised, discuss it with him, explain how you feel, but also give him the chance to explain and make sure you listen. It is only through talking and listening that we understand each other, and in that way you will build a successful relationship.