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Not so very long ago I remember two close relations offered to be sterilised so they could continue their relationship....the offer was refused and the male subsequently committed suicide.<<

This is not an argument in your favor. It poses far more questions than it settles. BTW From a Biblical perspective the story of Adam and Eve and Noah, if taken literally, imply that a lot of incest happened in the booting and rebooting of humanity. Aslo in Noah's case speaking of what nature gave us, incest must have been rampant in the repopulation of the unclean animals.