I have actually understood that concept as young as when I was 16. Most people can go through a moment of weakness. I don't know how to explain this without disclosing parts of my past I'm not proud of so I'll just go ahead: I remember I arranged for my best friend, who was an accomplished pua back then, to hit on my girlfriend, after finding out she still thought about him significantly more than she did about me. I understood how powerful these techniques could be though, especially in his hands, so I figured that if the two of them kissed it wouldn't prove anything as it would be just a moment of weakness, and I figured that anyone can have those. So my hypothesis condition was not whether they would kiss or not, which I took as a given, but whether she'd break up with me, which would prove non-repentance, rational thought, and planning future actions.
For that reason I'm not harsh on the wife. She was swept by the moment, and then she regretted what had happened while being dumbfounded at the reason she acted like that. Personally I would prefer dating women who are more in control of themselves, more willing to break social expectations and more assertive on doing what they want and avoiding what they don't, and such a woman would be very unlikely to go through a moment of weakness (and if it happened I'd consider break up not because of cheating but because of what it says of her as a person) but if she truly regretted it I'd stay with her, provided it didn't happen again. Women exist who like having short trysts with other men for fun. Women exist who would hold a long, involved affair in parallel. Those can't be explained by "spur of the moment", and would be actually very serious.
Eh, it is not necessarily applauding, turning a blind eye is enough. And girls are not around with their parents all the time. By the way, Steubenville and Maryville, among other places, are more extreme examples of what a culture might accept as permissible or not and the effect it can have in whole populations, within American culture. Not everywhere is like that, but it shows pressure does exist of acting a certain way, and people who stray from that are socially punished, sometimes very harshly. I'm not a woman living in an Anglo culture, and neither are you, but we have several posters who are one, and if they attest that such a pressure exists I have no reason to doubt their word
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To build may have to be the slow and laborious task of years. To destroy can be the thoughtless act of a single day. - Winston Churchill