is it normal to have thoughts like i do

Maybe she got caught up in a moment, felt flattered by attention, maybe she felt a little trapped by the whole marriage thing and needed a mini breakout.. Who knows.
But you stayed with her and you say that you have forgiven her, im sorry to say this but if you had truly forgiven her then you wouldn't keep punishing her by regurgitating the story searching for answers to questions that you should either have asked years ago or got answers that you doubted or couldn't accept.

None of us are perfect even if we think we are and even the tiniest patch of grass can look greener from the other side, she saw and lay on the grass but came back to you.

Be grateful that she did, if at the time, she could have foreseen the mental distress you are bestowing on her by going over and over things many years later, she should have run a mile and left you back then.

Ok she made a mistake, you should have moved on and the fact that you keep throwing this in her face says more about you than it does about her. Forgive, forget and move on or get out and both of you rebuild your lives.