In Michigan, adultery has long been a felony.

But when a judge warned that unfaithful spouses technically could be sentenced to life in prison, an obscure and seldom-used provision of the state’s criminal law became the subject of international scrutiny.

It’s unclear how serious Judge William Murphy of the Michigan Court of Appeals was when he pointed out the possible consequences of extramarital sex. Some observers say the liberal judge was making a political point by taking a strict interpretation of the law to an absurd conclusion.

Others have suggested Judge Murphy was trying to embarrass Michigan Attorney General Mike Cox, whose office triggered the ruling by appealing for a harsher sentence for a man who traded drugs for sex. In 2005, Mr. Cox acknowledged having an adulterous relationship.

Judge Murphy’s adultery bombshell was a footnote in a November ruling on a drugs-for-sex case. But since a Detroit Free Press columnist wrote about the footnote recently, online Web logs and radio talk shows have debated the pros and cons of life sentences for cheating spouses.

The ruling came in the case of Lloyd Waltonen, 43, a man from Charlevoix, in northern Michigan, who supplied a cocktail waitress with the prescription painkiller OxyContin in exchange for sex. Last year, Charlevoix Circuit Judge Richard Pajtas sentenced Waltonen to four to 20 years in prison but dismissed four counts of first-degree criminal sexual conduct, punishable by a life term, on the basis that the sex was consensual.

The state attorney general’s office successfully appealed Judge Pajtas’s ruling, citing an obscure provision of Michigan’s criminal law, which states that a sexual act committed at the same time as a felony constitutes criminal sexual conduct.

An appellate panel found Waltonen guilty of criminal sexual conduct. He has asked the state Supreme Court to consider an appeal of the ruling.

In the opinion, Judge Murphy wrote that although legislators may have drafted the law conceiving of scenarios in which there was a violent felony involving forced sex, he was “curtailed by the language of the statute from reaching any other conclusion.”

Judge Murphy wrote that a person was technically guilty of first-degree criminal sexual conduct any time he or she “engages in sexual penetration in an adulterous relationship.”

He noted that state law defines first-degree criminal sexual conduct as sexual penetration involving another felony. Because adultery is a felony, he wrote, adulterous sex could result in life imprisonment.

Such a sentence is unlikely. No one in Michigan has been charged with adultery since 1971.

Nevertheless, defense attorneys across the state are snickering and speculating about the prospect of life in prison for the attorney general.

From his office in Lansing, criminal defense attorney Hugh Clarke Jr. chuckled as he contemplated the idea — apparently raised by colleagues — of setting up a special prosecution team to charge Mr. Cox.

“It’s all so silly,” he sighed. “I only wish Judge Murphy would have used a different example. The judiciary in Michigan shouldn’t be held up to ridicule because of his use of that analogy.”

Mr. Cox declined to speak to reporters about Judge Murphy’s ruling. His spokesman, Rusty Hills, said Mr. Cox’s adultery was not relevant to the case.

“The case was about an individual trading sex for drugs,” Mr. Hills said. “A lot of people seem to be focused on a footnote at the very end of the case. Well, there are a lot of footnotes in a lot of cases.”

Law professors in Michigan looked on incredulously as the story was picked up by newspapers in Canada, Britain, and Germany.

“It’s a lot of hoo-ha about nothing,” law professor Richard Friedman of the University of Michigan said, describing the footnote as a throwaway point, a perfectly straightforward application of the law.

Judge Murphy’s footnote prompted heated discussion about the Michigan Supreme Court’s recent shift toward strict, literal interpretations of the law. Since 1999, when a Republican majority took control of the state’s Supreme Court, judges have been urged to interpret statutory language adopted by the Legislature literally. Some legal experts believe that Judge Murphy was making the point that such strict interpretation could yield absurd results.

John Jacobs, an appellate defense attorney based in Detroit, said Judge Murphy’s footnote took strict, literal interpretation to its logical conclusion: If judges could not apply common sense, legislators’ poorly worded laws could have bizarre ramifications.

“If you give that much power to legislators, they had better be right,” Mr. Jacobs said. “Every comma had better be right.”

One of the main complaints against such literal readings of the law is that prosecutors could potentially pick and choose whom they wanted to charge, said Judge Pajtas, who threw out the sex charges last year.

The appeals court ruling, Judge Pajtas said, had a range of implications, not just for unfaithful spouses: It might, for example, apply to a couple who broke into a building to steal liquor and decided to have sex before leaving the building, or a husband and wife who had consensual sex while using cocaine.

Tom Robertson, executive secretary of the Michigan Prosecuting Attorneys Coordinating Council, said there was no need for lawmakers to refine their definition of criminal sexual conduct. Although the law could theoretically cover anyone who had sex when they committed a felony, he argued there was no evidence of a prosecutor abusing the law.

“Just look at how the law was applied in this case: this person addicted someone to powerful narcotics, with a plan to extort sex from her,” he said. “That’s pretty gross.”