Affirmative-consent advocates say that rape prosecutions don’t produce very many prisoners. They cite studies estimating that fewer than one-fifth of even violent rapes are reported; 1 to 5 percent are prosecuted and less than 3 percent end in jail time. Moreover, Stephen J. Schulhofer, the law professor who co-wrote the model penal code, told me that he and his co-author have already recommended that the law do away with the more onerous restrictions that follow from being registered as a sex offender.

I visited Mr. Schulhofer in his office at New York University Law School to hear what else he had to say. A soft-spoken, thoughtful scholar and the author of one of the most important books on rape law published in the past 20 years, “Unwanted Sex: The Culture of Intimidation and the Failure of Law,” he stresses that the draft should be seen as just that — notes from a conversation in progress, not a finished document.

But the case for affirmative consent is “compelling,” he says. Mr. Schulhofer has argued that being raped is much worse than having to endure that awkward moment when one stops to confirm that one’s partner is happy to continue. Silence or inertia, often interpreted as agreement, may actually reflect confusion, drunkenness or “frozen fright,” a documented physiological response in which a person under sexual threat is paralyzed by terror. To critics who object that millions of people are having sex without getting unqualified assent and aren’t likely to change their ways, he’d reply that millions of people drive 65 miles per hour despite a 55-mile-per-hour speed limit, but the law still saves lives. As long as “people know what the rules of the road are,” he says, “the overwhelming majority will comply with them.”

He understands that the law will have to bring a light touch to the refashioning of sexual norms, which is why the current draft of the model code suggests classifying penetration without consent as a misdemeanor, a much lesser crime than a felony.

This may all sound reasonable, but even a misdemeanor conviction goes on the record as a sexual offense and can lead to registration. An affirmative consent standard also shifts the burden of proof from the accuser to the accused, which represents a real departure from the traditions of criminal law in the United States. Affirmative consent effectively means that the accused has to show that he got the go-ahead, even if, technically, it’s still up to the prosecutor to prove beyond a reasonable doubt that he didn’t, or that he made a unreasonable mistake about what his partner was telling him. As Judge Gertner pointed out to me, if the law requires a “no,” then the jury will likely perceive any uncertainty about that “no” as a weakness in the prosecution’s case and not convict. But if the law requires a “yes,” then ambiguity will bolster the prosecutor’s argument: The guy didn’t get unequivocal consent, therefore he must be guilty of rape.

SO far, no one seems sure how affirmative consent will play out in the courts. According to my informal survey of American law professors, prosecutors and public defenders, very few cases relying exclusively on the absence of consent have come up for appeal, which is why they are not showing up in the case books. There may be many reasons for this. The main one is probably that most sexual assault cases — actually, most felony cases — end in plea bargains, rather than trials. But prosecutors may also not be bringing lack-of-consent cases because they don’t trust juries to find a person guilty of a sex crime based on a definition that may seem, to them, to defy common sense.

“It’s an unworkable standard,” says the Harvard law professor Jeannie C. Suk. “It’s only workable if we assume it’s not going to be enforced, by and large.” But that’s worrisome too. Selectively enforced laws have a nasty history of being used to harass people deemed to be undesirable, because of their politics, race or other reasons.