It's great that Jeff Jacoby brings up the question of adult consensual incest, but he missed an opportunity to educate about marriage. Siblings are not allowed to marry, because marriage is a license to procreate, and siblings are not allowed to procreate (hence their being thrown in jail for incest). Marriages that are found to be between siblings are immediately declared null and void, even if they have no children and don't intend to have children, because only couples that have a right to have children together are allowed to be married.
The interesting question then is do same-sex couples have a right to have children together? The technology to create a child genetically related to two same-sex parents is currently being developed. In experiments in mice, it took 457 tries to get one mouse, named "Kaguya", to survive to adulthood, with the other 456 all dying at various stages along the way (10 were born alive but died shortly afterwards). I feel that the risks to the child are far too great, and will always be far too great, to allow this sort of experimentation in humans. Same-sex procreation must be prohibited, along with all other forms of procreation that do not combine a man's sperm and a woman's egg. Civil unions could be created to give committed same-sex couples, and perhaps even siblings, most of the rights of marriage, but they should not be given the right to attempt to procreate together that is the right of every marriage.