A taboo for most cultures is infidelity. Today in most western nations, it’s not even that big of a deal, it’s so widespread. No longer even grounds for a divorce. Women through the ages have pretty much had to turn a blind eye to it, since until the last couple of generations, most women were financially dependent on their husbands and the thought of just walking away wasn’t really feasible, who would look after them? So they tolerated it and looked the other way.
In ancient cultures it was a sin punishable by death and pretty miserable ones at that, everything from being dragged to death by horses, to starvation, to men being castrated and women being physically disfigured by having their noses cut off! If those were the punishments, you can see why you’d be less apt to do it. Having the job that I do and hearing about the miserable, loveless marriages that I do on a daily basis, I don’t blame most people for straying outside their relationships. I think monogamy isn’t natural, and to try and hold people long term to it is a recipe for disaster and just plain unrealistic in many cases long term.
In some cultures it is still punished harshly. Of course religion is often the base of such a taboo, the whole to death do you part and thou shall not covet they neighbors wife and all of that. I’m not advocating swinging and swapping, that seems just as unnatural, to share those details and experiences with a partner. It seems more natural to cheat on the sly…