The face-off on “Fox News Sunday” between Tony Perkins, the leader of the Family Research Council, and former solicitor general Ted Olson on gay marriage was illuminating. One could see precisely why gay marriage opponents are losing the national debate.

The Supreme Court building in Washington. (Karen Bleier/Agence France-Presse via Getty Images)

Perkins was asked how he would draw the line between a ban on  interracial marriage and one on same-sex marriage. He came up with two answers: Marriage is about children and gays cannot procreate, and gay marriage will lead to polygamy. (“First off, marriage is not to affirm adults. It’s for the protection of children. And if love is the only factor, where you do you draw the boundary?”) Olson demolished both arguments:

The purpose of marriage is what the Supreme Court has said 14 times. It’s a fundamental right that involves privacy, association, liberty, and being with the person you love and forming a part of the community and being treated equally with the rest of society. . . . What court after court after court has said, that allowing people of the same sex to marry the person that they love, to be part of the community and to be treated equally, does no damage to heterosexual marriage.

Then Perkins tried: “They can’t — there’s nothing in nature to say that’s normal.” And that is what it boils down to. The religious right as an article of faith believes homosexual unions are not “normal,” and it therefore wants the state to enforce that belief. But that, of course, is imposing religious creed by state action. As much as the religious right might believe it is not “normal,” that belief is a function of religious faith; it is not an argument that is going to find resonance with a court applying secular constitutional standards.

Perkins wasn’t willing to embrace fully the slippery-slope argument:

PERKINS: Let me ask you, what are the boundaries, though? If it’s just love, what are the boundaries? Where can we go with marriage?

WALLACE: What are you suggesting? That [there’s] going to be polygamy. That people will be marrying their pets?

(CROSSTALK)

PERKINS: No, I didn’t say that. If we remove the natural established boundaries for marriage, the union of a man and woman, we have removed those boundaries, those guardrails.

What the anti-gay marriage forces are lacking is a compelling argument that homosexual marriage harms heterosexual marriage:

WALLACE: We’re going to get to that in a second. But your argument as to whether somehow this damages the [Perkinses] to have another couple next door?

OLSON: Well, everyone who has ever talked about this says there’s no heterosexual couple that is going to decide to get divorced or not to get married or not to raise children just because another couple next to them is treated equally and with respect and decency under our Constitution. That is why we have courts.

The same argument Mr. Perkins was making was made with respect to interracial marriages in 1967 — 30 some states at one point prohibited interracial marriages.

And talk about the color of the skin? People were making the same arguments. Marriage is wrong between people of different races. We have to stop that.

When the Supreme Court finally acted, 16 states were still prohibiting interracial marriages.

In sum, gay marriage opponents have lost the argument with the public and the courts because what was once a matter of defending social consensus has evolved into a plea for enforcement of one set of religious norms in a diverse society. Without evidence of harm to others, there is no constitutionally acceptable reason to preserve the distinction.

Perkins gamely tried to invoke all sorts of stray arguments — from sex education curriculum to baking wedding cakes for gay couples — which are distinct issues and do not go to the essence of the legal issue at hand (e.g. parents can opt out of sex education for their children, matters of accommodation for religious beliefs are resolved on a case-by-case basis).

Believers have every right to set parameters on their own religious ceremonies and conceptions of marriage; but they cannot impose them on others — absent any evidence that they are being harmed by others’ marriage practices. In short, it is the essence of tolerance in our society that people of faith can abide by their own religious definition of marriage while tolerating others’ practices with which they strongly disagree. Whether they agree with that or not, that is the legal reality going forward. They will need to come to terms with it.