It all comes down to moral integrity and principle.
A promise of fidelity was made and, likely, expressed as one of THE most important factors in the relationship. Regardless of context, the PROMISE was broken, which means her word becomes meaningless.
If at once, "I promise not to do x" is broken, then you can't trust anything else the person promises to do or not do. Age, circumstance, doesn't really have much significance. Once a cheater, always a cheater - it's a one-way street.
OK guys - on this one I can shed some person experience. Many have probably heard this before.
I am in the process of getting a divorce. My wife cheated at with at least two other men, and both guys were wimps. The second one is possibly the ugliest guys I've ever seen. He is also about 3" shorter than the she-devil. And they are both narcissist, were both married (meaning he was too), both school teachers, and both were also both employed at the church I used to attend. In other words, they lead praise in front of our congregation, while committing adultery. I'll let the Lord handle that part.
Her "first" affair - if it was her first - was 10 years ago. In fact, it was consummated 10 years ago this coming Friday. It was awful and I was shattered. She never really showed remorse, but did cry - but only in front of other people. It was an act.
Why did I stay? I have 3 kids. At the time they ranged in age from 13 to 5. I did what I thought I had to in order to hold my family together. I stopped making things about me. But I now know that I also stopped having any passion in my life, and I died inside, and I tried to forgive her. She was never trustworthy.
I lived my life as a knight in rusty Armour - meaning I protected myself from allowing anyone to get close enough to hurt me like that again. I simply existed. It sucked.
Well, as I mentioned above - she did it again. We are getting a divorce. That is good - except for the financial part. FYI - if you live in a no-fault state and make more money than they person you are divorcing - mark my words - you will get RAPED financially. I don't mean treated unfairly - I mean RAPED. Just as if they drove a Class A Motor home up your ass, and with the mirrors sticking out.
My sons are devastated and won't talk to their mother. They knew about her first affair and knew about the 2nd one before I did. My ex-wife stopped being a mother to my daughter a few years before, and started confiding in my daughter as a best friend - although she forgot to tell my daughter she was screwing someone else. So now my 16 year old daughter feels responsible for her mother's emotional well being, and wants nothing to do with me (we are changing that with a counselor's help).
To the question: Stay or Go. I stayed, but now know I should have left. But, I'm glad I did everything in my power to be a man for my family, and that is the reason I tried to live in a shitty marriage for 10 years. The facts speak for themselves. I did the best I could, and she was a selfish pig.
I can never have those 10 years back, or the 17 that came before them. But, as Dave Grohl says in "Monkey Wrench":
One last thing before I quit
I never wanted any more than I could fit
Into my head I still remember every single word
You said and all the **** that somehow came along with it
Still theres one thing that comforts me since I was
Always caged and now Im free