the law regarding incest was only passed down by God during
moses time. and that was some time before adam and eve.
incest was forbidden by God at that point of time, because the
gene pool was starting to break down. that's why people who
practice incest, their kids are usually, for the lack of a better
word, fucked up. deformities, low IQ, watever.
and it's the same reason why if you have kids with someone from
another race, as far from the original gene pool as possible. the
kids are really good looking etc.
it's true that adam and eve and their kids commited incest, but
because at that time, the gene information was full, there was no
chance of "bad information" passing down the generation.
until moses time.
This is a dangerous position to take, IMHO.
Suddenly the moral reason disappears to be replaced with the
genetical reason.
And people can always wriggle their way out and say this is "not
so bad". For example, look at Josef Fritzl. How many of his
creations are of the farked up type because of genetical reason?
AFAIK, none. (One boy was hunched 'cos the dungeon ceiling is too
low)
Luckily Judaism does not take your position. The incest law was
passed down even before Moses.