So says Jeff Jacoby. I'm skeptical - not because the right to incest doesn't arguably follow from the logic of gay marriage, as Jacoby says, but because I think the demand for marrying one's sister is far too low to overcome the "ick" factor involved. The gay population is small, but not that small - even at 2-4 percent of the American population, it's large enough to create both a mass constituency for gay marriage and a still-larger percentage of Americans who count homosexuals as their friends and neighbors, and understandably wish them happiness as a result. Whereas even if the incest taboo begins to fray, I think the number of would-be Ptolemies and Cleopatras is so vanishingly small that Americans - including Supreme Court Justices - will never have much of an incentive to put the logic of Lawrence v. Texas ahead of their repugnance.
Moreover, the case for gay marriage appeals to Americans' sense of fairness in a way that the argument for incest doesn't. If gays can't (or shouldn't) marry straights, the pro-gay wedlock argument runs, then they deserve to marry someone. Whereas a straight guy who wants to marry his sister isn't just asking for the right to marry the kind of person he's attracted to - he's asking for the right to marry a specific person, and that's more easily refused.