Barry, the most likely way it happened given how you described you and your wife and your marriage, is that your marriage was routine and boring, though contented and happy. Not the "in love" infatuation butterflies in the belly going on anymore, not constantly telling her how hot she is, and if you were, she was so used to it that it was practically invisible to you.
Let's face it, "boring" usually means "no fights, no arguments," no "rocky" relationship, happy together, happy with your great kids, really you were what I refer to as living the dream. She never thought of cheating until this guy came and initiated.
I personally never met someone married 10-plus years who were madly and passionately "in love."
This guy told her how hot she was and she liked hearing that, he told her how much he liked her and she liked that, he told her how great a personality she had and she liked that, he told her how much she wanted her and she liked that, and over and over and over she NEVER told the guy FUCK OFF, instead she said, "pshaw, you shouldn't say that" but never complained that she was being sexually harassed when he kept doing it, and over and over and over she started to like it so much, she started the feelings of "in love" butterflies in the belly, etc. The guy is divorced, probably dates, so he is maybe used to knowing the right words to say.
And if she told him at all about your marriage, whatever negatives there might be, he played it up even more that he would NEVER do that - e.g., your wife said, "my husband fell asleep watching TV last night," and he replies, "with a hot woman like you, I'd never fall asleep with you around." You get the idea, I hope.
In my situation, I had all messages between my wife and other man, and he started with a few comments here and there, and when she ignored it, he upped it more and more, and then she started to respond, and this escalated in about two weeks before I could see my wife was "into it" and from there it went fast.
She was in full control. Her cheating was a choice. A conscious decision.
As Walloped posted, this is true. She knew what she was doing was wrong, but she did it anyway.
[This message edited by wk55hn at 10:20 PM, February 7th (Tuesday)]