Well, says Mervyn Silverman, suppose someone opened a Russian- roulette parlor, where adults so inclined could go risk their lives. Could the city be indifferent?

Silverman recently resigned as director of health in this city, where there is currently a death a day due to AIDS (acquired immune deficiency syndrome). Two new cases are reported each day, which means that a year from now there will be two deaths a day.

This city is -- depending on your idea of civic virtue -- famous or notorious for its tolerant attitude toward homosexuality. Ten to 15 percent of the population (75,000-100,000) are male homosexuals. Among many such men, unlike among lesbians, there is a pattern of promiscuity.

Bathhouses, featuring private rooms and saunas or whirlpools, are relics of a generation ago. Before homosexuals felt able to "come out of the closet," such establishments were used for assignations by people who felt they had nowhere else to do what they were determined to do. Today, Silverman says, bathhouses are less used for that, although they still are used by, for example, bisexual married men or others who desire secrecy. But bathhouses are symbols of "homosexual rights," so there was protest last April when Silverman promulgated regulations to prohibit "unsafe sex" in bathhouses.

San Francisco's yeasty political process stopped the regulations, so Silverman ordered the bathhouses closed. The owners "won" in court, but the court, in allowing them to remain open, essentially imposed Silverman's "unsafe sex" regulations, requiring monitoring of activity, forbidding private rooms and requiring lights to remain on.

Silverman argues that because of AIDS (and some less harmful diseases) bathhouse sex is inherently unsafe. Bathhouses are frequently used for forms of group promiscuity that should not be described in a newspaper.

To persons who say that the regulations will merely change the venue, not the quantity or nature, of homosexual activity, Silverman says: if couples meet in separate locations, the quantity of especially dangerous contacts will decline because group sex will decline. He says this is already happening under the tutelage of death: watching a friend die is educational. When we see a wreck along a highway, Silverman says, we drive more slowly for the next few miles, then soon speed up. But AIDS has a more lasting deterrent effect.

San Francisco spends several million dollars a year on education and counseling about sexually transmitted diseases. Information is, Silverman thinks, the primary reason for the changed behavior that has resulted in a 75 percent reduction in cases of rectal gonorrhea.

Sexually transmitted diseases are paradigms of many of today's principal public health problems, such as traffic accidents, cancer from smoking, alcoholism. They are "optional" diseases in the sense that they can be radically reduced by the dissemination of information that modifies freely chosen behavior.

Silverman is opposed by libertarians who say the regulations violate "civil rights." They say that publicity about AIDS has been so effective that almost everyone is informed, and that sexual activity in private between informed and consenting adults, even when dangerous, threatens only the consenters and thus is no business of government.

Silverman argues that even if the facts about AIDS have passed the threshold of public understanding that can be called "common knowledge," there are still victims who have never given "informed consent" to the risk of infection. Victims include women who have sexual relations with bisexual men, drug addicts who use tainted needles and recipients of tainted blood.

But the argument for Silverman's policy is most interesting when it moves beyond nuances about informed consent, and beyond the sort of argument used to justify laws requiring seat-belt use, or requiring motorcyclists to wear helmets. (If a cyclist scrambles his brains and becomes an invalid, his act is not just, as philosophers say, a "self-regarding" act. Rather, it is "other-regarding" because, thanks to the insurance industry and extensive government involvement in the provision of medical resources, we have socialized the burdens that result from individual irrationality, however consensual.)

The form of ethical argument natural to Americans and favored by libertarians favors libertarianism. It turns on strict individualism -- the individual's information, the individual's consent and the individual's rights, including his right to have government share the burdens he incurs. Such an argument skews policy debates because it excludes communitarian concerns, especially concerns for community values.

The soul of Silverman's argument is that the city is not interfering with anyone's right to commit any sexual act, but only with the bathhouse owner's right to facilitate dangerous sexual activity. He says the reason the city is right to interfere is the same reason that a civilized city would close a Russian-roulette parlor: life is good, and the law, a powerful teacher for good or ill, should affirm the preciousness of life by discouraging behavior that cheapens it.

Is this an argument for "legislating morality"? Yes.