On the contrary, it is the only natural solution that makes sense. If God gave a soul to two persons, would you not think that their offspring would naturally, by continued biological attraction, mate with the humanoids around them? (And yes, Adam and Eve might well have mated just with each other, while their offspring did not.)
The only alternative is that God miraculously removed them from their humanoid brethren after ensoulment, and miraculously had the descendents overcome the natural inclination against inbreeding and incest, and miraculously prevented the spread of genetic defects from inbreeding.
Now that sounds unnatural and ad hoc to me.
I would like to answer this from the position of Catholic teaching which is
designed
to bring all created people into a harmonious relationship with our Creator.
Adam had a specifically designed purpose as the first human. Somehow, we get so caught up in our own little worlds, that we forget that Adam’s future descendents–that is the whole human race which was to follow from his loins-- was destined to live in eternal love with our Creator.
The glitch was that Adam was not equal to God.
The first moment of Adam’s existence, at the dawn of human history, was in perfect friendship with God. This did not include bestiality by mating with non-human beings. Eve was created in the same nature as Adam and thus only she was a suitable partner.
The Garden of Eden is seen as the location where Adam and Eve lived in friendship with God. Whether it was allegorical or whether it had a zip code is not the point. The point is that this was the
time Adam had to freely make a choice to live as a spiritual creature in free submission to his Creator or to leave and go out on his own as another god. The rest is history.
Because future humans would be implicated in their founder’s reward for obedience, they became implicated in Adam’s original sin. The whole human race is in Adam “as one body of one man.” (St. Thomas Aquinas,
De Malo 4, 1.) Either way, God had blessed Adam and Eve saying “Be fertile and multiply…” (
Genesis 1:28) Adam called his wife Eve because she became the mother of all the living. (
Genesis 3: 20)
The key question is –
When did mating between siblings become wrong?
Starting with God’s blessing which would be in effect no matter if Adam remained in God’s friendship or not, the Catholic teaching is that Adam had received original holiness and justice (harmony of human nature) not for himself alone, but for all human nature. (CCC 404 -406) When death entered the human world, human nature was no longer in genetic harmony.
Shifting to basic science regarding genetics and heredity, there is the simple principle that genes can be dominant or recessive. There is also the simple observable principle that over time genes did change as sickness and death entered the world. For example, genes directing the immune system can grow stronger in some instances and weaker in other situations. After original sin, transmission of genes through propagation was not in harmony. Changes in the chemical base pairs (mutations) would occur as each new generation succeeded the previous generation.
By understanding genetics, one can come to the conclusion that the ill effects of mating with relatives would eventually come into being. This could be expressed by the degree of relationship. Thus the question of “incest” between siblings remains as a question of timing.
Ah, one says. Adam’s great-grandchildren did not know anything about genetics. True. But they did have eyes and could see what was happening. And they were rational beings so that they could put two and two together.
What the knowledge of genetics tells us, is that the children of Adam could mate with their siblings without much trouble. Given the length of female fertility, there would be overlapping generations which would enlarge the “degree” of relationship. In practical terms, the need for siblings to mate with each other would be very short, shorter than the time for genes to cause hereditary diseases.