When you hear “zero tolerance,” what do you think?

Until very recently, good liberals gave the term a bad rap. It conjured draconian school-discipline rules, which led to a disproportionate number of suspensions for students of color. And it resonated with the broken-windows philosophy of Rudy Giuliani and other tough-on-crime politicians of the 1990s and early 2000s, who sought the maximum penalties for even the most minor violations.

But that was then, and this is #MeToo. In the struggle against sexual harassment, zero tolerance has suddenly become the accepted liberal dogma. Miscreant students deserve second and third chances, just like turnstile jumpers and squeegee men do. But for sexual misconduct, it’s one strike and you’re out.

That’s a huge mistake. School discipline and public safety were not served by suspending or locking up as many people as possible, regardless of their infractions. Likewise, the campaign against sexual misconduct will suffer if we fail to distinguish between different kinds of it.

Don’t tell that to New York Sen. Kirsten Gillibrand, who spearheaded the effort to persuade her fellow senator Al Franken to resign. “I think when we start having to talk about the differences between sexual assault and sexual harassment and unwanted groping, you are having the wrong conversation,” Ms. Gillibrand said, defending her call for Mr. Franken to quit. “You need to draw a line in the sand and say none of it is OK. None of it is acceptable.”

She’s right, of course. But some forms of it are worse than others and deserve greater penalties; some aren’t as bad, and they should be punished less. That’s called the principle of proportionality, which lies at the heart of any reasonable system of justice. And once you do away with it, you can do anything to anyone. ...