There ought to be a law," according to the old expression, but sometimes it would be far better if one wasn’t necessary.

That comes to mind in the case of Jamie and Tess Meier, a couple from Maui who demonstrated Aug. 21 for gender equality as part of a National Go Topless Day.

They picketed outside Haleiwa Beach Park and then on a Waikiki sidewalk, said their lawyers, and both were bare-chested except for strategically placed pieces of tape.

In Honolulu, there is no law that strictly prohibits being topless, so police instead used an ordinance that regulates gatherings in public places to issue a citation, with the penalty up to a $500 fine and jail time.

The protest itself may not inspire much of a rallying cry — it’s doubtful that there’s much general support for shedding bikini tops at public beaches here, and there are far more urgent gender-equality issues that need addressing.

Even so, the Hawaii office of the American Civil Liberties Union correctly objected to the police action as one that could undermine constitutionally protected rights of free speech and assembly, and city prosecutors are right to review the issue.

The ACLU also makes a compelling case that the underlying ordinance should be revised to make it clearer when exercising those rights should require a permit.

Here’s the wording of the ordinance: "Any person using the recreational and other areas and facilities under the control, maintenance, management and operation of the department of parks and recreation shall first obtain a permit from the department" for a range of uses, including "meetings or gatherings or other similar activity held by organizations, associations or groups."

Whatever city lawmakers meant by "groups" — it’s undefined in this instance — calling a husband and wife a group seems a stretch.

More important, allowing police to use this ordinance to quash a legal protest sets an uncomfortable precedent.

This doesn’t mean there were any easy regulatory options for police. City ordinances against indecent exposure and open lewdness deal with exposing genitals, not breasts, and disorderly conduct rules require a trigger that goes beyond mere toplessness.

Under Section 711-1101 of state law, prosecutors would have to prove the "intent to cause physical inconvenience or alarm … or recklessly creating a risk" by one of a range of behaviors.

These would include fighting, threatening or exhibiting "violent or tumultuous behavior," making "unreasonable noise," subjecting another person "to offensively coarse behavior or abusive language … likely to provoke a violent response," "creating a hazardous or physically offensive condition" without authorized license or permit, or impeding or obstructing any person in a public place by begging or soliciting alms.

That’s a fairly high bar — and it should be.

Neither does it seem wise simply to add breasts to the list of verboten body parts. Nursing mothers could be affected by that, and it could prompt a legitimate challenge of gender discrimination, because men are allowed to be shirtless.

Government has a proper role to play here, though it’s not in legislating behavior. Especially with the approach of the Asia-Pacific Economic Cooperation meeting, there should be advisories on where police will draw distinctions between legal protests and public disturbances.

As for behavior like the Meiers’, freedom of speech goes both ways. People who find this display offensive could simply walk up to them and tell them so, or, more pointedly, file a police complaint. At some point, unwelcome expression can be construed as a public affront, but not if people don’t speak up.

The truth is this protest was an isolated event, not a real problem. Most people will back down when told they’ve crossed some kind of line.

That’s common courtesy, which still exists, even if its power to keep things civil in public places has been, in recent history, underrated.