I can state unequivocally that this "once/always" concept does not apply to everyone. From personal experience.
I cheated on my last girlfriend. Happily, as I recall. It was like sitting down and playing a challenging video game. You see, I was unhappy in my relationship, I was unfulfilled, I was going through tough times. At the same time, though, I was loath to end the relationship, because my insecurity was kicking in. It became an elaborate game...making time for both women (and even a third!) and not getting caught, inspired deceptions, intricate lies. Eventually, the fun became a drudgery, and even then I knew it really wasn't fair to anyone involved. Eventually, my girlfriend broke up with me...to be honest, I would've broken up with me. Starting an real relationship with the 'other woman' turned out to be a non-starter, as I should've assumed it would, and a month later, my future wife and I met. We have been together ever since that first night, bound at the hip. Eight years. And, I've been a good boy, not even an inkling of becoming a bad one. We're both justifiably proud of this marriage.
So, this "once a cheater, always a cheater" thesis is a crock of shit.