The first law of thermodynamics says that energy can't be created and can't be destroyed—it can only be changed from one form into another. The same holds true of the puritanical impulse.

Puritanism in the historical sense is as dead as the Salem witches. The religious group that settled in New England outlawed theater, rejected any form of sex except marital intercourse, banned celebration of Christmas and spent hours in church listening to horrifying depictions of Hell.

But the term has come to be a synonym for any disapproval or discouragement of carnal pleasure. Sexual puritanism has receded even among devout Christians, who generally see nothing wrong with husbands and wives gratifying each other however they please.

In society as a whole, things have changed even more drastically. Virginity is no longer held up as an ideal for young people; TV has an abundance of flesh and raunch; and the majority of Americans no longer see homosexual acts as "always wrong."

Most people don't think it's their place to tell others what sort of sexual behavior is acceptable. With few exceptions, it has become a private matter of individual preference. Laws against sodomy are extinct. Divorce is easy to get. Your sex life is off-limits to government regulation. Busybodies have little impact on policy.

But puritans haven't vanished. They've merely changed the subject. The expansion of freedom in matters of sex has coincided with a shrinkage in matters of health. New Yorkers would laugh at laws policing sex, but they elected a mayor who has no problem trying to control other physical indulgences.

New York City Mayor Michael Bloomberg brought forth a ban on large sugar-laden beverages, which this week was struck down by a state court. But the idea won't go away that easily: The city will appeal the ruling, and other cities are considering similar laws.

Nor is this approach out of character for Bloomberg, whose attitude is: "Let my conscience be your guide." He prohibited restaurants from using transfats, banned smoking in bars and restaurants as well as most outdoor spaces, compelled fast-food chains to post calorie counts on their menus, proposed limits on sodium and even stopped hospitals from giving bottles of infant formula to new mothers. When it comes to what you put in your body, nothing is off-limits to the city.

The sugary drink measure has been controversial, but if experience is any guide, it will someday be as common and accepted as smoke-free taverns. Individuals could be allowed to make their own choices without coercing others, but that doesn't satisfy the public health zealots.

Many of them yearn to limit nicotine in cigarettes, at least until they can ban them entirely. Many would like to curb fast-food outlets as well: Los Angeles has blocked new ones from opening in some neighborhoods. Several local governments, including Boston and Philadelphia, have pronounced transfats verboten.

The usual rationale is safeguarding health, which Bloomberg said would save taxpayer dollars that go to treat residents afflicted with lifestyle-related diseases. His lawyers argued that sugary drinks promote obesity, which is to blame for many of the 500,000 or more New Yorkers who have diabetes, each of whom average an extra $6,649 a year in medical costs.

But the underlying motive is to enforce one model of acceptable behavior on everyone. Obesity is commonly regarded as a grave personal failing, an abdication of healthy restraint and abstinence. Some of the virtuous feel entitled to demand virtue of all.

Sound like anyone who landed at Plymouth Rock? Truth is, sexual puritans can make equally plausible arguments on the practical need to regulate the exercise of bedroom behavior, which has major implications for both health and government budgets.

Non-marital sex, after all, produces unwanted births, including among teens who will become public burdens. It increases the incidence of abortion, which some states cover under Medicaid. It spreads diseases that can have fatal consequences, including AIDS and human papillomavirus.

But in the realm of these fleshly pleasures, we have learned to let people make their own decisions, even if they have some impact on others. We've largely liberated ourselves from government interference into deeply personal choices that are a central part of what makes us human.

Well, some of them, anyway. Escaping the puritans is a task that is never finished.