Incest

While I am on the topic of irrational social rules and laws: why are incestuous marriages still forbidden? Is there any reason for this, except that the number of people who would be interested is very small, the number of people who think it's disgusting is very large, and no politician would be caught dead tackling the issue?

Yes, of course it increases the chance of genetic disease. But, first of all, not all people have children, and there is no point of refusing marriage for eugenic reasons to people who can't reproduce anyway, such as gays and hetero couples where the woman is post-menopausal. Second, if a brother and a sister decide to have a kid, they can do it without the benefit of marriage, and there is not a damn thing that anyone can do to stop them. Third, and most important: this society - any Western society - is currently extremely opposed to the idea of the state telling people not to have children - let alone not to marry - for some eugenic purpose. A couple where both partners are carriers of Tay-Sachs are not prohibited from having children, let alone marrying. Women who have HIV are allowed to have children even though the chance that the kid will be positive as well is very high. Even the people who conceive 7 fetuses at a time due to misuse of fertility drugs and then refuse to abort some of the fetuses are not forced to do so, and mostly not even ostracised much. So why is a child of siblings such a scary thing that we have to ban them from marrying just in case?