I don't think there's any way of determining how detrimental and affair is except to look at the specific situation as you try and solve the issues in the relationship. An affair is a symptom of problems in the relationship - it's detrimental to both parties in terms of desire to resolve the real issues, and it can do real damage to the relationship if both parties can't get over it and move on.
There is a good deal of information on this website and if you do a google search you will find countless articles, many of which conflict with others. You need to use your wisdom and intuition to help you take the kernels of truth from each that apply and overlook what doesn't. Similarly, you need to understand that no matter what you know, it won't change the situation. You won't be able to control the actions of another person. The DB method is built on the premise that both parties have to want to work at the relationship, and through changing ourselves wwe encourage the desire to work on the relationship in our WASs. This is where you have to face your demons. People don't like to change, and you have to work hard to get to the core of your issues. You may need a counselor (this is advisable because you're likely hurting because of the situation), but be sure that this counselor is on board with your goals for self improvement and how this intersects with your desire to save your marriage.
Notice I didn't say that this is how you intend to do so, because you need to acknowledge the need for lasting self improvement, and your changes have to be driven by your will to improve. This will improve your marital situation, but it is something you have to be dedicated to regardless of whether you are going to save your marriage.
Time does play a big part in this. In a lot of affair situations, the affair goes on undisclosed and ends on its own. Time is your friend, but don't just sit back and wait, live your life, love yourself.
“No problem can be solved from the same level of consciousness that created it. ” – Albert Einstein