"If the Supreme Court says that you have the right to consensual sex within your home, then you have the right to bigamy, you have the right to polygamy, you have the right to incest, you have the right to adultery. You have the right to anything…. The definition of marriage has not ever to my knowledge included homosexuality. That’s not to pick on homosexuality. It’s not, you know, man on child, man on dog, or whatever the case may be."

For those words, Sen. Rick Santorum, R-Pa., has been widely denounced by many Democrats and some moderate Republicans as a raving homophobic bigot. And it was indeed offensive to so casually associate the millions of decent people who happen to be gay with practices almost universally condemned as immoral.

But Santorum’s remarks are more plausible as legal analysis, in the context of a major gay-rights case now before the Supreme Court, than most critics have acknowledged. Meanwhile, Santorum’s assertion in the same April 7 interview that there should be no constitutional right to privacy reveals an agenda that is more controversial than most of his defenders seem to understand.

The issue in Lawrence v. Texas, which the justices heard on March 26 and will probably decide by late June, is whether to strike down as unconstitutional a Texas law making it a crime to engage in homosexual sodomy. The answer should be yes, and I think it will be. "To criminalize any enjoyment of their sexual powers by a whole category of persons is either an imposition of a very great cruelty or an exercise in hypocrisy inviting arbitrary and abusive applications of the criminal law," in the words of Harvard Law professor Charles Fried, a conservative who was President Reagan’s solicitor general.

But there are at least two ways to strike down the Texas law. And the narrower way would not lead toward the slippery slope of which Santorum warns.

The broad way to strike down the Texas law would be to hold that constitutionally protected privacy includes "the right to be free from governmental intrusion into, and criminalization of, private sexual relations between consenting adults," as the Human Rights Campaign and more than 25 other groups argue in an amicus brief.

Santorum has a point in asserting that such a ruling could put us on a slippery slope toward legalizing some forms of incest, the most repellent of the practices he listed. Indeed, by 5-4, the Supreme Court held in 1986, in Bowers v. Hardwick, that if the Court were to recognize an unqualified right to "voluntary sexual conduct between consenting adults, it would be difficult, except by fiat, to limit the claimed right to homosexual conduct while leaving exposed to prosecution adultery, incest, and other sexual crimes" within the home.

To be sure, consensual sex does not violate laws against adultery, bigamy, or polygamy unless it breaches a contract of marriage, which is a state-created privilege conditioned upon a vow to have sex only with one’s spouse and to have only one spouse at a time. And incest is not consensual in the bulk of cases, which involve parental rape or seduction of minor children.

But what about what one law professor delicately calls "Lord Byron’s problem"-consensual sex between an adult brother and sister? (In Byron’s case it was a half-sister.) While that practice would, unlike gay sex, often risk producing children with serious birth defects, the risk is non-existent for infertile women and can be eliminated for others by vasectomy, sterilization, contraception, or abortion. Besides, privacy advocates (and the Court) would surely reject the notion that states can bar people with heritable defects from having children.

Might Lord Byron’s problem nonetheless be outlawed because it has long been despised as repugnant and immoral? Not if there is an unqualified constitutional right to consensual adult sex. And sodomy, no less than sibling incest, has for centuries been subject to "condemnation … firmly rooted in Judeo-Christian moral and ethical standards," as the late Chief Justice Warren E. Burger stressed in a concurrence in Bowers v. Hardwick. Public opinion is moving toward majority support for a right to have gay sex. But it’s not there yet.

So Santorum’s legal analysis was not as outlandish as many have suggested. Indeed, he would have been on the mark had he phrased it like this: If the Supreme Court said you have the right to consensual sex within your home, it would be a giant step toward a right to commit incest with your adult brother or sister and (at least if you are unmarried) to have group orgies.

Many libertarians would counter that the Court should recognize such a broad freedom-of-consensual-sex principle, just as it has long recognized freedom-of-speech principles broad enough to protect hard pornography and hate speech. I would agree. But I suspect that the justices will avoid such a sweeping expansion of the right to privacy, which is rooted in the due process clause. One reason is that-because the Texas law forbids gay couples from engaging in sexual practices that are lawful for heterosexual couples-the Court can strike it down on narrower, equal-protection-of-the-law grounds. Another is that headlines such as "Court Legalizes Sodomy and Incest" would generate a more forceful backlash than "Court Bans Discrimination Against Gay Sex."

The narrow way of striking down the laws criminalizing homosexual (but not heterosexual) sodomy in Texas and three other states would be to rule that even though rarely enforced, they seek to condemn a whole class of millions of gay Americans to lifelong, involuntary sexual abstinence, and to stigmatize them as criminal deviants. Laws criminalizing Lord Byron’s problem have no comparable effect, because few if any people can hope to find sexual fulfillment only with their siblings. And such a ruling could avoid any discussion of the laws in nine other states that ban heterosexual and homosexual sodomy alike. (To be sure, these laws affect gays disproportionately and could be struck down for that reason.)

The argument for striking down the Texas law is far stronger than was the Court’s basis in 1996 for using the equal protection clause to void a Colorado ballot referendum that would have barred enactment of any state or local laws against anti-gay discrimination in employment, housing, and public accommodations. (It would have also repealed all existing gay-and-lesbian-rights laws.) That 6-3 decision, Romer v. Evans, held that states cannot "make [gays] unequal to everyone else," and that the Colorado provision "lacks a rational relationship to legitimate state interests" because it "seems inexplicable by anything but animus toward the class that it affects."

Santorum, a devout Roman Catholic, said in the April 7 interview that he is driven not by animus but by his conviction that "this freedom"-which I read to mean the freedom to engage in any sex outside of traditional marriage-is "antithetical to strong, healthy families." There may be a modicum of truth here. But in an era of rampant adultery, divorce, unhappy marriages, spousal violence, child abuse, deadbeat dads, premarital sex, birth control, out-of-wedlock births, and working couples whose kids spend most waking hours in day care centers, it seems far-fetched to list gay sex as one of the real threats to the traditional family.

Speaking of slippery slopes, Santorum’s assertion on April 7 that the "right to privacy … doesn’t exist in my opinion in the United States Constitution" would take the law in a direction far more objectionable than would the "right to anything" that he is so worried about. By explicitly rejecting Griswold v. Connecticut, the 1965 decision that established the constitutional right of married couples to use contraceptives, Santorum suggested that states should be free to ban contraception, period. And his apparent goal of overruling the entire 80-year line of right-to-privacy decisions would not only be a step toward allowing states to criminalize all premarital and extramarital sex; Rick Santorum’s America might also be a place in which states would be free to mandate sterilization of convicts and retarded people.

Santorum did not mention those issues specifically. But the right to privacy that he assails was the main basis for the Court’s rulings in Skinner v. Oklahoma, in 1942, banning forced sterilization of certain convicts; and Pierce v. Society of the Sisters, in 1925, guaranteeing parents the right to choose private or religious schools for their children. (Pierce could be, but was not, based on the First Amendment independent of the right to privacy.) How would Americans like to see those decisions overruled?