Dave Winer tries to offer an objection to a common argument against gay marriage — that if you allow gay marriage to happen, incestuous marriages between consenting adults are bound to follow,
There are good technical reasons for disallowing marriage between relatives for genetic reasons. But if they chose to marry with the stipulation that there would be no offspring, why should any of us object?
Uh, no, actually, there aren’t good reasons — at least ones that are consistent with other values that we have about marriage and reproduction.
It is true that allowing closely related people, such as brothers and sisters, to marry and reproduce does increase the risk of having a child with a congenital birth defect. As Wikipedia notes,
There is a popular belief that incest results in physically or mentally deformed children, although this is a vast oversimplification. Inbreeding (which may occur through incestuous or non-incestuous relations) results in an increase in homozygocity, that is, the same allele at the same locus on both members of a chromosome pair. This occurs because close relatives are more likely to share more alleles than nonrelated individuals. If an individual has an allele linked to a congenital birth-defect, it is likely that close relatives also have this allele; a homozygote would express the congenital birth defect. If an individual does not have such an allele, a homozygote would be healthy.
In small populations this dynamic would lead to an initial increase in birth defects. But if health care is limited, it is likely that such children would not reproduce; consequently, the frequencies for the allele in question would go down. Ultimately the result would be a population with a large number of homozygotes and a small number of congenital birth defects. In large populations with good health care, however, it is likely that there will be consistently high levels of heterozygosity despite periodic inbreeding. Consequently the alleles linked to congenital birth defects will remain in the population, with a significant chance of a homozygote with the linked allele.
But this is not much of an argument against such marriages. There are people, for example, who know they are carriers of much more potentially severe genetic diseases who are allowed to marry and have children.
For example, somebody who carries the Huntington’s Disease gene has a 50 percent chance of passing that gene on to his or her children. And yet, we do not require people to be tested for Huntingon’s Disease before marrying, nor do we require people who test positive for the HD gene to refrain from having children should they marry.
Similarly, no one, to my knowledge, has suggested encouraging carriers of Fragile X to agree not to breed, much less requiring that they agree to do so before being allowed to marry.
Surely if preventing homosexuals from marrying while allowing heterosexuals to do so is a violation of the equal protection clause — as defenders of gay marriage typically insist — then barring relatives to marry due to genetic reasons while allowing non-relatives with known genetic defects to marry is also a violation of the equal protection clause.