Well, we're not discussing what was normative in all cultures and times. The Ballads under discussion arose in 'Christian' communities of the British Isles where incest was taboo, not in ancient Egyptian (or indeed ancient Chinese) where it was not.

Much the same situation might be said to be found in more recent times with Catholic Church in many poor countries - whereby boy children were/are used sexually by older more powerful individuals within a community which while outwardly rejecting such practices, yet knows and turns a blind eye to it.