Its not a strawman. Its basically exactly what same-sex marriage supporters say they believe in. Over and over. For example from this very thread:
Are a brother and a sister, both over 18, two consenting adults. Absolutely.
Why? Because it embarrasses same-sex marriage advocates and/or reveals them as massive hypocrites?
EVERY argument made in favor of same-sex marriage works equally well for incestuous marriage.
This is why, as you already know, it's an issue:
http://en.wikipedia.org/wiki/Incest
Inbreeding
Main article:
Inbreeding
Incest that results in offspring is a form of close
inbreeding (reproduction between two individuals with a common ancestor). Inbreeding leads to a higher probability of
congenital birth defects because it increases that proportion of zygotes that are
homozygous, in particular for deleterious
recessive alleles that produce such disorders
[103] (and see
Inbreeding depression). Because most such alleles are rare in populations, it is unlikely that two unrelated marriage partners will both be heterozygous carriers. However, because close relatives
share a large fraction of their alleles, the probability that any such rare deleterious allele present in the common ancestor will be inherited from both related parents is increased dramatically with respect to non-inbred couples. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes. However, because the increased proportion of deleterious homozygotes exposes the allele to
natural selection, in the long run its frequency decreases more rapidly in inbred population. In the short term, incestuous reproduction is expected to produce increases in spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects.
[104]
There may also be other deleterious effects besides those caused by recessive diseases. Thus, similar
immune systems may be more vulnerable to infectious diseases (see
Major histocompatibility complex and sexual selection).
[105]
A 1994 study found a mean excess mortality with inbreeding among first cousins of 4.4%.
[106] Children of parent-child or sibling-sibling unions are at increased risk compared to cousin-cousin unions. Studies suggest that 20-36% of these children will die or have major disability due to the inbreeding.
[15] A study of 29 offspring resulting from brother-sister or father-daughter incest found that 20 had congenital abnormalities, including four directly attributable to autosomal recessive alleles.
[107]
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