Why did my girlfriend take another man's number about hiking?

My girlfriend openly confessed to me she took a man's number at the gym who asked her to go hiking with him. My fiancée prior that I split with 5 years ago had cheated on me physically and emotionally and I have ZERO tolerance for any type of deceit.

My girlfriend said she made a mistake blah blah but I can't seem to let it go. I'm at the point where I want to break up with her. Am I being too extreme?