<human>: Can you give me the gist of the text in a nutshell?

Over the past weekend I came to learn about 3 separate situations of infidelity among associates of mine (just friends of friends) and it made me rehash the painful memories of learning my ex-fiancée was cheating on me.  
  
My question goes out to mutual friends of both sides who learn of these "secrets". Why do *some* of you feel it is none of your business and choose not to inform the other person (especially when you are close to both)? I am forever grateful to my ex-fiancée's best friend who told me the truth and spared me the mistake of marrying a woman who cheated on me with multiple men (and I later found out married another man a year later and started cheating on him).   

I even recently learned about a co-worker who's knows that his best friend cheated on his wife multiple times before getting married, and has not slowed down one bit. He tells me he loves both his friend and adores his wife and just wishes he would stop because his wife is the sweetest woman in the world. The only thing that kept going through my mind was anger that a wonderful woman is being wasted on a cheating husband and wanted to shout out, "Why the fuck is no one telling her!?"  
 
I firmly believe these lies never stay secret forever and will only hurt the other person the longer it takes for them to find out.
<bot>: As a mutual friend to a couple, when you find out the other is clearly cheating on the other, why do some of you stay quiet?