Because keeping the peace is generally easier than keeping relationship promises and both are important to people. So Western society’s developed ideas around physical privacy that are stronger than hiding fantasy violence. Seeing violence doesn’t (generally) make you want to do the same. Where as nudity and titillation etc generally stoke an appetite for sexual variety. And a lot of people don’t generally enjoy their partner looking elsewhere, nor physical comparisons. So we end up with these cultural values: nudity / sex is a bit taboo whereas violence gets a pass.
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