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If we all descended from one man and one woman then it's quite apparent that a third generation was not possible without the sexual union of close relatives.
Does the Bible (or other Christian source) specifically address incest? Is it a sin?
If it is not a sin then does it follow that it is not practiced in Christianity for purely secular reasons?
If it is a sin should the children of Adam and Eve not have reproduced?
The OT clearly condemns in Leviticus 18 and 20 ("If a man takes his sister [...] and sees her nakedness [...] it is a disgrace [...] and he shall bear his iniquity."). This is reinforced at other later points, but we do not find record or such a prohibition earlier.
Clearly there is no way that the line from Adam and Eve continued for at least the first few generations without some in-marrying.
We know from science today that in-breeding makes for bad genes. Perhaps in the early days when things were a little bit less degenerate physically (and people were living to their 900's) God allowed such a practice, but as the gene pool grew and there was no need for it, he introduced a prohibition for our own good. Clearly today it would be a sin of disobedience against God.
It is written in Leviticus 18:6 that it is a sin -
None of you shall approach any one of his close relatives to uncover nakedness. I am the LORD.
and in 1 Corinthians 5:1 -
It is actually reported that there is sexual immorality among you, and of a kind that is not tolerated even among pagans, for a man has his father's wife.
The reason that it was not a sin for the children of Adam and Eve was at the time, the law had not yet been established.
As regards the children of Adam and Eve, the narrative of Cain suggests in at least three places that there were other people on Earth at his time:
First, in Genesis 4:14-15, he worries that he will encounter people who will want to kill him, and God gives him a mark of protection.
Second, he then moves to Nod, at which point he has a wife (Gen. 4:16-17). The wife is presumably not a child of Adam and Eve, as we do not hear of any other children being born after Abel until Seth, which is mentioned after six generations from Cain (Enoch, Irad, Mehujael, Methushael, Lamech, and Lamech's sons, Gen. 4:17-25). The birth of Seth comes when Adam is 130 years old, and daughters and other sons followed on after that (Gen. 5:3-4), while Cain is implied to be born very shortly after the Fall.
Third, Cain builds a city in Nod (Gen. 4:17), which assumes a community of some size already existing.
The only place in the creation narrative where it suggests only two humans were ever created is Genesis 3:20, "Now the man called his wife's name Eve, because she was the mother of all the living"—and this in itself does not look like a literal statement, as at that point she had not yet, in fact, given birth to anyone.
(Certainly it looks like only two humans were put in Eden; in the Elohist account, though, we have that male and female humans were created on the sixth day (Gen. 1:27) and their number is not specified, at least in the usual English translations.)
You are all over-complicating things. Here's how it happened:
God created Adam, and then Eve. (Genesis 1:27-28; 2:22)
God then told them to be fruitful and multiply (make love and have children) (Genesis 1:28) [Note that Genesis tends to jump around a little chronologically]
Now, the gene pool then was so pure (sin isn't here yet) that Adams offspring were able to interbreed and not have any defects at all. Presumably to the point up until God outlawed it in Deuteronomy 27:22-23 and other passages. Obviously the gene pool started to grow causing more and more defects with each inbred child.
It's clearly a sin because:
God outlawed it in the OT
And it is apparently appalling to Paul when he learns of a man sleeping with his mom in 1 Corinthians 5:1, thus implying that the act is still outlawed and not acceptable in Gods eyes.
It is, indeed, a sin, as the ones who answered before me have pointed out it is said in the Scripture. But incest was permitted from the creation of man until the giving out of the laws to Moses. In the time before that, humanity was in its 'young age'.
It's like when a parent only imposes the first rules on his child when he is old enough to need and to understand them.
