In 1954, with the debate over the legality of public school segregation at its height, Fred Sullens of the Jackson Mississippi Daily News offered a vision of the future to come. "White and Negro children in the same schools," he wrote, "will lead to miscegenation. Miscegenation leads to mixed marriages and mixed marriages lead to mongrelization of the human race."

Proponents of racial equality were, naturally, not eager to endorse any such conclusion. The 1954 Brown v Board of Education decision said nothing about marriage rights, and neither the early Civil Rights Acts of 1957 and 1960 nor the landmark 1964 Act tackled the then-widespread state-level bans on interracial marriage. To do so would have been highly impolitic. And given the abysmally low levels of public approval of interracial marriage in the 1950s and 1960s it is reasonably likely that many sincere supporters of civil rights did not particularly desire the "mongrelization" outcome Sullens foresaw.

And yet in retrospect, he and other naysayers were entirely correct.

Correctly understanding this history is important today as controversy swirls around a new "religious freedom" law in Indiana that could create a special religious exemption to nondiscrimination laws. Liberals see it as simple bigotry, while conservatives see the backlash as part of a growing illiberalism on the part of the American left. The truth is that conservatives are often more correct than liberals allow about the reality of slippery slopes — but also too eager to base policy conclusions on this fact.

The racial justice slippery slope

While the initial claims of the civil rights movement were broadly libertarian in nature — arguing that the government itself should stop discriminating on the basis of race — it very swiftly went beyond that. By the early 1960s, the claim was that the government should actively prohibit private discrimination in both employment and the provision of services. This move from curtailing unfair government policies to having the government regulate private enterprises was one of the causes of the historic swing of the Republican Party from the pro to the con side on civil rights debates.

By 1970, the IRS was going further still and ruled that schools that practiced racial discrimination could not qualify as tax-exempt nonprofits.

Several schools at different levels sought refuge in the First Amendment's free exercise clause, arguing that it was a question of religious belief and that denying them the same tax exemption offered to other religious schools was itself a form of discrimination. But in 1983 the Supreme Court ruled otherwise, holding that Bob Jones University's ban on interracial dating was a valid cause for denying it tax-exempt status.

The conservative fear about marriage equality

As controversy swirled this weekend over Indiana's controversial new Religious Freedom Restoration Act, I saw many liberals share Ian Milhiser's February 2014 article "When ‘Religious Liberty’ Was Used to Justify Racism Instead of Homophobia."

It's an excellent piece.

But correctly understanding this history explains not only why liberals find the idea of a religious right to discriminate so unpersuasive, but also why religious conservatives find the opposition to such laws so alarming.

Andrew Sullivan, the intellectual pioneer of the marriage equality movement and also someone who tends to lean right of center on economic issues, almost always refers to "civil marriage equality" as his goal. The point is to recognize that marriage is, in part, a religious ceremony and that religious officials should be free to conduct religious unions in accordance with the dictates of their faith. Many rabbis will not perform marriages between mixed-faith couples, and Catholic churches generally refuse to host second marriages of divorced people. By the same logic, a church should be permitted — as a religious matter — to decline to recognize same-sex marriages even while the state grants equal treatment to couples regardless of gender.

It's about a lot more than cake

In this view, the real fear is not so much that bakeries will be forced to make cakes for gay weddings as that the slope will continue to slip.

Indeed, conservatives who were not necessarily all that fired up about the Indiana law in the first place are alarmed by the liberal backlash to the law.

A man who made his name controlling supply chains in China thinks religious liberty bad for business. Huh. — Michael B Dougherty (@michaelbd) March 30, 2015

That's in part because social conservatives simply don't take the interests of gay and lesbian couples as seriously as liberals do. But it's also because the specter of giant business enterprises rushing to condemn Indiana shows how far and fast the slope slips. This Twitter exchange between Matt O'Brien and two conservatives illustrates both sides of the argument. Liberals feel that if "religious liberty" can justify anti-gay discrimination, then the case for bans on racial discrimination unravel. Conservatives feel that if conscience does not allow service providers to choose their customers, there is no stopping tyranny of the majority.

Maybe someday soon, opposition to same-sex marriage will be seen as equally beyond the pale as Bob Jones's ban on interracial dating. Maybe churches will be forced to choose between losing their tax-exempt status and being forced to perform weddings they regard as theologically illegitimate. This strikes me as far-fetched — the Catholic Church and other religious institutions have, for decades, managed to secure exemption from bans on gender discrimination in employment even as formal gender discrimination in general is deemed unacceptable. But though it's far-fetched, I've seen enough history that I won't call it crazy. It turns out the crazy Mississippi racist was right and the sensible civil rights advocates were wrong about the slope from school integration to race mixing.

Politics is about choices

The question here is ultimately more difficult than most liberals have allowed. Personally, one reason I'm not paranoid about religious institutions that discriminate against gays someday having their tax exemptions threatened is that I think the tax exemption for religious institutions is bad policy, and I'd be open to extending that to secular charities, too.

But that would be a long way for the slope to slip, and it's certainly not what opponents of the Indiana law are arguing.

On the other hand, the conservative quest for pristine "limiting principles" to govern nondiscrimination law is in important ways misguided. Politics is all about making choices, including choices about where to draw lines, including choices that are somewhat arbitrary. Why is the speed limit 65 miles per hour rather than 67? Well, because it's a round number. So why is the Social Security eligibility age set to rise to 67 rather than to 70? Well, because we wanted to be slightly more generous to elderly people than would be required by 70. Why is it illegal to refuse to serve a customer on the basis of his race, but perfectly legal to practice implicit racial discrimination in whom you date? Why do we ban gender discrimination in employment but allow for single-sex schools? Why do religious people get conscience exemptions from some laws but not all laws?

The answer, ultimately, is that policymaking requires judgments about what is practical and what is important. Conservative fears about slippery slopes are sensible, but the debate ultimately is about discrimination in serving gay clients and isn't about far-fetched hypotheticals. Liberals are saying that making gays and lesbians feel more comfortable and accepted in their communities is more important than making homophobic service providers feel more comfortable and accepted in their communities. That's as legitimate a thing to disagree about as anything else.

But to win the argument, conservatives will have to persuade America that they are right on the merits. So far, it seems like they're failing.