The “slippery slope” is a popular argument in the same-sex marriage debate.

Where do you draw the line, opponents argue? If you start allowing marriage between people of the same sex, then why not require that the law recognize threesomes, group marriage, incest, beastiality, and polygamy? Can “crazy cat ladies” fulfill their dream of legal marriage to their favorite tomcat? Once you start tweaking with meanings, is marriage “done fer,” in my dad’s phrase?

The slippery slope argument is emotionally persuasive, because most all of us can imagine some sexual universe that we absolutely, uncategorically do not want to see happen. Maybe for feminists it’s The Handmaid’s Tale; maybe for social conservatives it’s free abortions for all.

We don’t want to be on a slippery slope that leads toward our personal dystopia, however we define it.

The jeremiad that our country’s going to hell in a handbag is engrained in America’s DNA, dating back to second-generation Puritans, perceived by their dismayed parents as a spiritual declension from their own pious benchmark.

The aspect of the slippery slope argument that I agree with is that mores do change over time.

While I see a slope, in the sense of movement over time, my first objection is that it’s not slippery, in the sense of being sudden, abrupt, or invariable.  

There are many speed bumps, warning cones, and debris that force us to use brakes on the would-be slippery slope of sexual mores.

One of the biggest is consent. Cats can’t consent to marry their crazy cat lady owners. Children can’t consent under age of consent laws to sex or marriage. Incest is prima facie not consensual. A change in the definition of marriage might invite slippery declension into unseemly or forbidden arrangements, but to legally sanction them, you’d also have to dispense with laws of consent, among many others, that delimit permissible unions.

Another speed bump on the slope is the consensus-building process that’s presumably central to a liberal democracy, although it’s a woefully muffled force today. A lot of citizens contribute to the definition of marital and sexual norms (Today, same-sex marriages co-exist with a culture of abstinence, purity balls, and 19^th-century revivals of “traditional” marriage). That heterodoxy helps to forge legal and social compromise.

Most citizens end up neither entirely satisfied nor entirely miserable with the outcomes of that process, which frustrates more radical or ideologically-entrenched citizens. But in any case, compromise decelerates change. It puts grippers on the slippery slope.

Second, the “slippery slope” implies change that’s out of our control, in which humans are swept downward by the inadequately-anticipated consequences of their own logic.

You don’t walk, amble, or march with sure-footed deliberation down a slippery slope. You flail down it, sliding ineluctably from Point A to sordid Point Z, as if you have no agency.

But our will, and desires, are two other major potholes on the would-be slippery slope.

I don’t think we go out and do things in life because they’re theoretically permissible within one interpretation of jurisprudence.  We do things, or advocate for things, that we actually desire to do.