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Yes & yes. Cultural taboos (apart from the purely biological ones) evolve as moral norms which serve tribal preservation purposes. Mate guarding/jealousy provides an obvious example, congruent with your point about nuclear families. In such arrangements, each parent = precious.
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This makes a lot of sense. I came across a Netflix show which had an episode on polyamory. With some tribal cultures, they treated kids as if they had many fathers. Though this likely wasn’t ubiquitous to all tribes, those that had this norm had no issues with non-monogamy.
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I wouldn’t think so. In all the traditional cultures I’ve learned about the marriages are decided by matriarch/patriarch and their marriage is linked to the credibility of whoever decided for them to match.
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I think it depends, really. In cultures without nuclear families, I think it may hurt slightly less but it's easier to walk away from. In cultures with nuclear families, it probably hurts more but the family endures it better.