Gut Instinct and Denial

What do you do when you have that gut feeling and even attempt to warm your spouse about someone who is obviously interested him/her?  All I got in return was, "No, it's not like that.  It's just a friend from _____". I completely trusted my spouse and was completely deceived. If I had listened to that instinct, I might have been able to stop the sexual affair before it happened.  It was probably too late for the emotional part.

I had been calring giving that advice for weeks before the affair happened. My spouse initially denied it. Once I found the evidence (explicit photos, emails, etc.), it was impossible to deny. I didn't keep a cool demeanor, but I don't believe that an adulterer deserves compassion upon discovery.  Especially considering the boldness of the lies and depth of deception that accompanied the betrayal.

But before that, I was trying to be honest and caring with my concern and trust my spouse blindly.  Maybe I shouldn't have.  Where did I do wrong?