It’s a little known fact that Martin Luther King Jr. didn’t really lead the March on Washington.

What actually happened is that the marchers, a quarter-million strong, grew impatient waiting for the event to begin and stepped off the curb ahead of schedule. When they found out what had happened, King and other march “leaders” had to scramble to catch up. Freedom was in the air and the marchers saw no need to wait for permission to move.

Forty-one years later, that vignette from another era offers an irresistible analogy to frame what has been happening these past few days in San Francisco. Public opinion seesaws between tolerance and intolerance, courts and legislatures debate civil union and marriage and abruptly, thousands of gay and lesbian couples decide to stop waiting for other people’s decisions.

San Francisco mayor Gavin Newsom makes the quixotic decision to issue marriage licenses to same-sex couples, and suddenly gay men and lesbians are rushing as fast as planes, trains and Nikes will carry them, to the city where Tony Bennett left his heart. Critics say the mayor has acted in defiance of state law, but Newsom calls same-sex marriage “inevitable.”

In the long term, he might well be right. In the short term, it’s a dicier question. The issue is being fought in the courts even as we speak and nobody can say with authority how it will come out. Gay marriage may move forward to legality, may move backward to prohibition; it remains to be seen.

The one thing that seems beyond debate is that the issue is indeed moving.

And there is, I think, something uplifting in the manner of that movement. Meaning that it comes not at the behest of some charismatic national leader or the bidding of some strident national organization. People are moving, rather, two by two, moving upon decisions made at dinner tables and in front of televisions, moving upon a conviction that now is the time.

Drops of water melting into a flood. The flood hurling itself against a wall, intolerance.

When you get past selective application of biblical injunctions and invocations of moral concern, that intolerance usually boils down to this curious bit of reasoning: Discrimination against gays ought to be allowed because, unlike skin color and culture, homosexuality is something people “choose” and thus, can un-choose.

So, critics say, society ought not be required to extend civil rights protections to gay people. Rather, gay people ought to be required to change.

The most absurd of the many absurd things about that argument is this: It asks us to believe a man might have his choice of a sexuality that is accepted and celebrated and one that will leave him open to ridicule, estrangement, physical abuse, job and housing discrimination, and the loss of basic legal protections … and he would take the second one.

If that’s not the dumbest thing I’ve ever heard, it’s definitely in the top 10.

Granted, science has yet to figure out what causes homosexuality. Its roots may be psychological, may be physiological or may be, as I suspect, a combination of both.

But ultimately, it doesn’t really matter, does it? Whatever causes a man to be gay or a woman, lesbian is obviously powerful enough that they have no real choice in the matter. The people who have been flocking to Mayor Newsom’s city did not decide to be gay. Anyone who is watching them with that thought in mind is missing, probably on purpose, the point.

What they have decided is that they are human beings worthy of human dignity. What they have decided is that they are tired of waiting for people to get that.

What they have decided is that it’s time to step off the curb.

Leonard Pitts Jr. is a columnist for the Miami Herald.