It was a common-sensical observation, though you wouldn't have known it from the nail-spitting it triggered in some quarters. When the justices, voting 6-3, did in fact declare it unconstitutional for any state to punish consensual gay sex, the dissenters echoed Santorum's point. "State laws against bigamy, same-sex marriage, adult incest, prostitution, masturbation, adultery, fornication, bestiality, and obscenity are . . . called into question by today's decision," Justice Antonin Scalia wrote for the minority. Now, Time magazine acknowledges: "It turns out the critics were right."

Time's attention, like the BBC's, has been caught by the legal battles underway to decriminalize incest between consenting adults. An article last month by Time reporter Michael Lindenberger titled "Should Incest Be Legal?" highlights the case of Paul Lowe, an Ohio man convicted of incest for having sex with his 22-year-old stepdaughter. Lowe has appealed his conviction to the Supreme Court, making Lawrence the basis of his argument. In Lawrence, the court had ruled that people "are entitled to respect for their private lives" and that under the 14th Amendment, "the state cannot demean their existence or control their destiny by making their private sexual conduct a crime." If that was true for the adult homosexual behavior in Lawrence, why not for the adult incestuous behavior in the Ohio case?

The BBC program focused on the case of Patrick and Susan Stubing, a German brother and sister who live as a couple and have had four children together. Incest is a criminal offense in Germany, and Patrick has already spent more than two years in prison for having sex with his sister. The two of them are asking Germany's highest court to abolish the law that makes incest illegal.

" We've done nothing wrong," Patrick told the BBC. "We are like normal lovers. We want to have a family." They dismiss the conventional argument that incest should be banned because the children of close relatives have a higher risk of genetic defects. After all, they point out, other couples with known genetic risks aren't punished for having sex. In any event, Patrick has had himself sterilized so that he cannot father any more children.

Some years back, I'd written about a similar case in Wisconsin -- that of Allen and Patricia Muth, a brother and sister who fell in love as adults, had several children together, and were prosecuted, convicted, and imprisoned as a result. Following the Supreme Court's decision in Lawrence, they appealed their conviction and lost in the Seventh Circuit Court of Appeals. Lowe will probably lose too.

But the next Lowe or Muth to come along, or the one after that, may not lose. In Lawrence, it is worth remembering, the Supreme Court didn't just invalidate all state laws making homosexual sodomy a crime. It also overruled its own decision just 17 years earlier (Bowers v. Hardwick, 1986) upholding such laws. If the court meant what it said in Lawrence -- that states are barred from "making . . . private sexual conduct a crime" -- it will not take that long for laws criminalizing incest to go by the board as well. Impossible? That's what they used to say about normalizing homosexuality and legalizing same-sex marriage.

In Germany, the Green Party is openly supporting the Stubings in their bid to decriminalize incest. According to the BBC, incest is no longer a criminal offense in Belgium, Holland, and France. Sweden already permits half-siblings to marry.

Your reaction to the prospect of lawful incest may be "Ugh, gross." But personal repugnance is no replacement for moral standards. For more than 3,000 years, a code of conduct stretching back to Sinai has kept incest unconditionally beyond the pale. If sexual morality is jettisoned as a legitimate basis for legislation, personal opinion and cultural fashion are all that will remain. "Should Incest Be Legal?" Time asks. Expect more and more people to answer yes.

Jeff Jacoby's e-mail address is jacoby@globe.com.