olorado prosecutors dropped sexual-assault charges against Kobe Bryant in September after his accuser decided she was unwilling to testify. But Ian Ayres of Yale Law School and Katharine Baker of the Chicago Kent College of Law contend that even the behavior Bryant admitted to -- unprotected consensual sex with a woman he had just met -- was irresponsible and dangerous. They have a proposal to curtail such behavior: outlawing ''reckless sex.''

Ayres and Baker define reckless sex as penetration, without a condom, in a first-time sexual encounter. Because such sex leaves behind forensic evidence, it would be relatively easy for prosecutors to prove that it had occurred. Anyone accused of the crime could then offer the defense that omitting the condom had been consensual. But he or she would have to prove this by a ''preponderance'' of the evidence.

Both men and women could theoretically be charged with sexual recklessness -- and sentenced to up to six months in jail. Women would have a fairly easy time defending themselves: a man's insertion of a condom-free penis would almost certainly demonstrate his consent to such an encounter.

Ayres and Baker say that raising a legal obstacle to first-time sex without a condom would reap benefits for public health. ''The lion's share of sexually transmitted infections are caused by first-time sexual encounters,'' they argue on the legal-affairs Web site Balkinization. Moreover, failure to wear a condom may amount to prima facie evidence of disdain for women: ''Few men careful enough to use a condom are reckless enough to rape. The same recklessness that causes men to overlook the risk of disease and pregnancy can also lead them to overlook whether the woman has truly consented.''

Of course, Ayres and Baker aren't doing away with the ''he said, she said'' problem as much as shifting it to the question of who did or didn't want the man to wear a condom. But they point out that men could avoid courtroom arguments over consent simply by wearing a condom that first time. The paper has raised objections both obvious (privacy) and subtle (what are implications for gay men?). But before you ridicule the proposal as a parody of the Nanny State, the authors ask that you keep in mind two things: unprotected sex can kill and date rapists almost always walk.

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