Even in supposedly liberal circles you are liable to be seen as some kind of weirdo or worse if you bring up this issue. The contrast with gay marriage could hardly be more stark.

Why is it banned? One rationale right now is the increased average risk of birth defects to children. But recent studies show that this risk is low and comparable to the risk faced by women having children over 40.

Would you support banning marriage by middle aged women?

Despite the near total neglect of this issue, the numbers of people affected worldwide are probably greater than that by gay marriage, since the mainly homosexual probably do not account for more than 5% of the population. 

The United States is the only Western country to restrict it, although certain industrialized nations like South Korea do also (but then in Korea it is often unacceptable to marry someone with the same last name). It is mostly legal in the liberal Northeast, defying stereotypes of Southerners marrying cousins. I'm happy to say that my home state of New Jersey is tolerant here.

How did these bans come about? They were not present at the time of the United States' founding, but only emerged after the Civil War as new states joined the union. Cousin marriage in these new states was seen as a primitive and undesirable practice. Ironically, many of the same undercurrents were present in the eugenics movement that surfaced in the United States near the same time. Both eugenics and the ban on cousin marriage sought to improve society by restraining breeding.

The genetic justifications for these bans have been shown to be hollow. Here is the beginning of the New York Times article:

Contrary to widely held beliefs and longstanding taboos in America, first cousins can safely have children together, without a great risk of birth defects or genetic disease, scientists are reporting today. They say there is no biological reason to discourage cousins from marrying.

First cousins are somewhat more likely than unrelated parents to have a child with a serious birth defect, but scientists say the risk is not large. In the general population, the risk that a child will be born with a major birth defect, like spina fida, is 3 to 4 percent; to that background risk, first cousins must add another 1.7 to 2.8 percentage points, the researchers said.

Although the increase represents almost a doubling of the risk, since the background risk is small to begin with, the result is still not considered large enough to discourage people from having children, geneticists say. And they point out that no one questions the right of other people with far higher levels of risk to have children. For example, people with Huntington's disease, a severe neurological disorder, have a 50 percent chance of passing the disease to their children.

Some highlights from later in the article:

"Somehow, this hasn't become general knowledge," Dr. Motulsky said. "Among the public and physicians there's a feeling it's real bad and brings a lot of disease, and there's a lot of social and legal disapproval."

"They are ancient laws in terms of thinking it's really bad," he said. "The data show it isn't that bad. There shouldn't be a law that you can't marry your cousin."

Finally, let's remember that Einstein and Darwin both married first cousins.

Let's hope that activists stop ignoring this issue.