First as a Six Day Young Earth Creationist, I take note of the fact that the Incest restrictions don't come until the time of Moses. And I believe Cain and Seth married their full Sisters as did all Adam and Eve's children.
Critics of The Bible love to view this argument as absurd. But Genesis 20:12 shows us that Abraham and Sarah where brother and sister. "And yet indeed she is my sister; she is the daughter of my father, but not the daughter of my mother; and she became my wife.". The later incest restrictions don't just generically condemn Brother-Sister incest, they specifically single out half siblings sharing the same father. And Abraham and Sarah is one of the most approved romantic relationships in The Bible. God specifically wanted Abraham's descendents through her, not anyone else, to inherit the Covenant.
Back to Genesis 4, some take
Either way though, I believe over a century had passed by the time Cain killed Able (about 9 months before Seth was born is my conjectured timing) Plenty of time for some of Adam and Eve's children to travel about, they were commanded to Fill and Populate the Earth from the
Another thing, Adam was basically married to his Clone.
The reason Genetic problems occur because of Incest is because of The Fall, Genetic mistakes always occur in the gene pool, and closer related individuals are more likely to have similar mistakes. But closer to when this deterioration started it was far less of a problem. I suspect God was preemptive and that it still wasn't until awhile after the time of Moses (Ussher dated it to 1492 B.C. I'm undecided on if I agree with Ussher's date, but Ussher's is the youngest I can accept) that noticeable problems would have been a concern.
All pagan mythologies have Brother-Sister incest (And other forms, but that's most common) among the gods. Even the mythologies of cultures that all through our recorded history of them disapproved of Brother-Sister incest, like the Greeks and Romans. I believe the reason this happens is that it's the distant memory of when Brother-Sister incest was not just allowed but nearly universal.
Now to get to the The Law.
Leviticus 18, Leviticus 20, and Deuteronomy 27 are the three passages listing Incest restrictions. Only L20 and D27 list punishments to be carried out, so only those were part of Israel's civil law code. L18 is what
Wikipedia, and I expect other places, accuse these
Wikipedia has a chart covering the various incest restrictions of the three chapters, but that chart made some mistakes, the one I addressed above and others, so I made my correction of it, and will share it with you all here.
Boxes filled in Red are condemned in that chapter, being still white and empty means it's not.
Step Siblings are ok. I don't know if that's illegal today, but people do have a tendency to consider it gross. It doesn't address anyone related by Adoption either, unless The Law simply doesn't consider adopted children/siblings distinct.
Some relations not allowed aren't biological ones, so genetic issues weren't their only concern. Inheritance and some other things were in mind too.
No same gender examples are covered, to some this would be because homosexuality is a sin altogether, but as I argued elsewhere, it's not. Genetic issues were not the only concern, but all the concerns did come basically down to reproduction in some way. So there was no reason to be concerned with same gender examples.
You'll notice they're not entirely gender neutral. The genders are different, including in how our Genetics work (Mitochondrial DNA is passed on only by the mother being one example). The main gender difference is, no restrictions on uncles marrying nieces, while nephews marrying aunts depends. Modern conventions are inclined to be the opposite here, if the relation implies the man is older then that's far more likely to be condemned as perverted. But as long as the younger is already an adult and consents I don't care.
It also doesn't address Cousins. Cousin relationships are inherently the most common (We're all Cousins ultimately if you believe The Bible) but First or even Second Cousin relationships tend to be viewed as wrong in most parts of the western world today. But at different periods this was different. The Bible not only doesn't outlaw Cousin relations, no matter how close, but it encourages First Cousin marriages in certain circumstances.
Most notably The Daughters of Zelophehad, discussed in Numbers 26:33, 27, 36, Joshua 17 and 1 Chronicles 7. Inheritance in Ancient Israel was normally Pater-lineal, but Zelophehad had no sons, so God decided in such a circumstance daughters could inherit so long as they married within their tribe, preferably their nearest kinsmen. They wound up marrying their first cousins.
Reheboam had many wives, all three named were fairly close cousins, and the mother of his successor was a first cousin. From what I argued before, pre-Moses examples don't prove anything, but I still feel like noting Jacob was married to two first cousins, and Issac to his first cousin once removed.
And if you consider Deutercanoical books like Tobit canon (as Catholics do). Well that is a book that in the form we have it in is practically making it a sin not to marry your Cousin. It seems the Book Tobit as we know it is a product of a time when Israelites were so paranoid about mixing with foreigners, they felt you should marry your nearest relation possible that's not forbidden in Leviticus 18. Reason why I specify "as we know it" is because I have my personal theory that in the original version Sarah wasn't Tobit's cousin, in fact I speculate her name means Princess for a reason. But that's for another topic on another blog. Point here remains that if you think Tobit as it appears in the Septuagint, Catholic Bibles, or the 1611 KJV is God's word (I don't) then you have a pretty firmly pro Cousin marriage text in your Canon.
Genetically, between any two people there is always at least a 2% risk of these kinds of genetic problems. Incest only increases the risk of it. First cousin relations (if there were few or no prior examples of incest in the shared ancestry) only increases the risk to 4%. So it's really not a big deal at all.
Today, for many reasons I don't think Gentile nations on this side of the Cross should be basing their Civil Laws on Ancient Israel's.
Genetic concerns are (or should be) the only concern of our modern civil laws. Some consider relations that imply a large age-gap should also be inherently wrong, but we have Statutory Rape laws (Which I feel need improving but I don't oppose having them) to protect minors. If both are adults and consent then I don't care and don't think the state should.
As a Libertarian I think only the most direct Heterosexual examples should be illegal, Biological Brother-Sister relations, and direct ancestors or descendants. But all incest laws outlawing anything else I oppose.
But to an extent I'm beginning to think perhaps any Incest laws are bad. The reasoning is the concern of genetic issues in the offspring makes it not a victim less crime. But that's basically Eugenics logic, and allowing the State permission to forbid reproduction in the case of Incest sets a precedent that could prove very dangerous.