The biological reasons came long after the moral reasons.
No. The biological problems with incest have always existed.
It is in fact because of those, that the morals concerning it, followed.
I submit that if incest had no negative biological side effect whatsoever, it wouldn't be a moral wrong at all.
Instead of brother and sister, consider cousins.
Still problematic biologically, but less obvious. And if you look at cultures around the world, you'll see that many of them don't have such moral objections to cousin pairing.
We regard incest as wrong not because of some abstract or emotional reason. But for a physical reason. Because it creates problems in off-spring.
It's a crime against the family in that to have the opportunity to join to another family and not do it is a sin against the social nature of man, and to forego four grandparents in favor of two is a sin against your children.
That is simply not true.
Again: look at cultures around the world. Plenty of cultures has no problems at all pairing cousins together.
In fact, there are plenty of cultures, like certain groups of jews, that have unique genetic problems among them, as a direct result of +2000 years of interbreeding within their family.
Again, the moral implication here is a direct result of the biological effects of reproduction through incest. The problems when brother and sister procreate are obvious and oftenly immediatly apparant. If you go to 1st or 2nd degree cousins, it is less apparant and will only start manifesting after a certain amount of generations - due to the lack of genetic variation.
This knowledge is what extended our moral objections to sibling pairing to also 1st and 2nd degree cousin pairing.
Then you don't have a sense of the deficiency of the polygamous bond in comparison.
Care to clarify?