When David H. Petraeus resigned as CIA director because of adultery, he was widely understood to be acknowledging a misdeed but not a crime. Yet in his state of residence, Virginia, as in 22 others, adultery remains a criminal act, a vestige of the way U.S. law has anchored legitimate sexual activity within marriage.

In most of those states, including New York, adultery is a misdemeanor. But in others — Idaho, Massachusetts, Michigan, Oklahoma and Wisconsin — it is a felony, though rarely prosecuted. In the armed forces, it can be punished severely although usually in combination with a greater wrongdoing.

This is yet another example of American exceptionalism: In nearly the entire rest of the industrialized world, adultery is not covered by the criminal code.

Like other state laws related to sex — sodomy, fornication, rape — adultery laws date to the Old Testament, onetime capital offenses stemming at least partly from a concern about male property. Peter Nicolas of the University of Washington Law School says the term stemmed from the notion of "adulterating" or polluting the bloodline of a family when a married woman had sex with someone other than her husband and ran the risk of having another man’s child.

Linda C. McClain, who teaches family law at Boston University, likes to give her students two decisions from New Jersey courts, the first from 1838 and the second from 1992, to demonstrate how things have changed.

In the 1838 decision, the court said that the harm of adultery lay not in "the alienation of the wife’s affections, and loss of comfort in her company," but in "its tendency to adulterate the issue of an innocent husband, and to turn the inheritance away from his own blood, to that of a stranger."

In the 1992 ruling, in a civil case, the court said that "adultery exists when one spouse rejects the other by entering into a personal intimate sexual relationship with any other person." It said it was "the rejection of the spouse coupled with out-of-marriage intimacy that constitutes adultery."

Most states have purged their codes of laws regulating cohabitation, homosexual sodomy and fornication — sex between unmarried adults — especially after the 2003 Supreme Court decision of Lawrence v. Texas that made sexual activity by consenting adults in private legal across the country. But the question of how that ruling affects adultery remains unanswered because others may be harmed by adultery — a spouse and children. Several courts have alluded to the constitutionality of adultery laws since the Lawrence decision.

But Melissa Murray, a professor of law at the University of California, Berkeley, said she thought "most courts in light of Lawrence are going to give adultery a wide berth."

Murray added: "It is an open question whether adultery continues to be viable as criminal law even though it remains on the books in 24 states and territories. Nobody is going to be going to jail for it. But it is used in divorce and custody cases and even in some employment cases."

A number of law professors, including Joanna L. Grossman of Hofstra University, said one reason that adultery laws remain on the books is that getting rid of them would require politicians to declare their opposition to them, something few would do. In addition, many like the idea of the criminal code serving as a kind of moral guide even if certain laws are almost never applied.

Petraeus is a retired four-star general who collects a military pension and remains subject to military codes of conduct that prohibit adultery. But Diane H. Mazur, a professor of law at the University of Florida and a former Air Force officer, said that the chances of the Army calling Petraeus back to active service in order to court-martial him over adultery are zero, as are any chances of state criminal charges being brought.

"That would be reserved for the most unimaginably serious circumstances," Mazur said. Even within the military code, she added, adultery is charged as a criminal offense only when "the conduct of the accused was to the prejudice of good order and discipline in the armed forces," she read from the manual for courts-martial. That meant something larger than seemed at stake here.

Murray said her research had led her to conclude that laws regulating sex emanated from a notion that sex should occur only within marriage. Criminal law, she said, was there to reinforce marriage as the legal locus for sex. So any other circumstances — sex in public or with a member of the same sex, or adultery — were all violations of marriage.

"Now we live in an age when sex is not limited to marriage and laws are slowly responding to that," she said. "But we still love marriage. Nobody is going to say adultery is OK."

© 2012 The New York Times Company