I think I read somewhere that in most cases, children who witness their parents' toxic behaviour either mimic it growing up or try desperately to do the exact opposite. Rarely do they act something "in-between"/"normal".
For example my parents were quite irresponsible and also drank&smoke a lot. From my early "childhood" (if you can call it that) until my early twenties, I never drank, smoke, was always the "control freak", "the responsible" one etc... Only when I got cheated on and then started doing IC did I move towards the middle, to let go of trying to fix everything, I started drinking alcohol (had a phase), though I never smoked. And ultimately I had to work real hard to stop myself from going totally in the opposite direction, of just drinking too much for too long, to be really irresponsible etc.
And I think it's like that also with experiencing your parents cheat - it messes you up, it's just a question of how it will manifest. And it can manifest itself first in the opposite direction, as you're trying desperately to not use the behavior patterns you're used to, and then sometimes you "break" and go to it.
So perhaps, like many WS mentioned here in this thread, you're trying desperately to tell yourself&everyone that you're never going to do that, but in the end, sometimes we end up doing what had been the most familiar and "normal" to us growin up, unless we had gone through some sort of counselling to deal with it.
And as others have said, a factor is also the subconscious "reconciliation" with your parents. After all, since you "all did it", it can't be that horrible of a thing, and your childhood therefore wasn't that bad, you were just "too young to understand".
And not only reconciliation, but in a way "devictimization" - they've been victims of cheating for so long, and now that they've cheated, it makes them feel less like victims. That was part of the reason why I in a way for a while after being cheated on, I struggled with the desire to either cheat on someone, anyone, or be the other man, just to be on the other side for a change.
So as contradictory as it seems, I can understand. But still, in my experience, it would have hurt less if my wxgf had no experience being cheated on or witness cheating. At least I could console myself that she was ignorant of the pain and devastation it causes. But her doing it despite being aware of it just made it seem more callous and a bigger betrayal.
Cheating, a gift for all the family.