God has set up at least two situations in which his creations had to sin in order to not go extinct. Of course, I’m referencing Adam and Eve giving birth to children who then had to have sex with either each other or their parents and Noah and his nuclear family who faced the same choice. The only moral thing to do for our ancestors, from the Christian perspective, was to let the species die off. In fact, since both times the need for incest applied to all but the most asexually reproducing creatures, they all had to sin or die.
Incest is a sin now, but wasn’t in Adam’s and Noah’s time. This gets God out of the position of creating something that he either wanted to die or disobey, questionable motivations for a loving father, but it means that sin is variable. It means that morality is not always constant. This notion throws a wrench into the apologetic premise that moral facts are absolute and moral values are objective.
Christian apologists tell me that certain things are morally right while others are morally wrong not because society defines them as such or even that they conform to God’s whims--but because they are facts of the nature of things. To them, God’s nature informs reality’s nature and God is unchanging. Assuming Christianity is true, incest switching values is profound. Does it mean God’s nature changes? No, it logically cannot. A “nature” is the way one is, without the subject deciding to be that way. If God’s nature changed, who are we saying changed it? They aren’t likely to say a greater deity and if they did, it would move this conundrum to that God. No, it means that the Christian God really does arbitrarily decide good and evil and, at least in this case, flipped the script. Why? Mysterious ways, man. Mysterious ways.