First the taboo against consanguineous marriages was most likely because of genetic diseases. Biology may not have been very advanced, but people were smart enough to pick up on patterns. Similar to taboos against cannibalism despite having no concept of prion disease.
If this was pushed by the catholic church then explain Italian families!
Is “the hero’s journey” not a story about rugged individualism that has been told over and over in different forms across cultures since the beginning of history?