The legal fight over same-sex marriage in federal courts is just beginning, and the outcome is far from certain. But in the aftermath of Judge Vaughn Walker’s ruling overturning California’s Proposition 8 banning same-sex marriage, what's striking is just how weak the legal arguments for upholding bans on same-sex marriage are.

Defenders of  same-sex marriage bans have to explain why people should not be allowed to marry whomever they choose. They have to explain why the law should discriminate between potential marriage partners solely on the basis of their gender. And they have to do it without relying on private moral or religious judgments about what is acceptable behavior.

The truth is that it will be hard to reconcile any ban on same-sex marriage with the Equal Protection Clause of the Fourteenth Amendment, which says that no state can “deny to any person within its jurisdiction the equal protection of its laws.” The amendment was attached to the Constitution in 1868 in response to the Black Codes enacted in many southern states limiting the rights of blacks to own property or enter contracts. The courts have ruled that under the Equal Protection Clause, laws that infringe a fundamental right on the basis of racial or ethnic identity have to meet a very high “strict scrutiny” standard that requires that they be “narrowly tailored” to serve a “compelling” government interest. Laws that make distinctions on the basis of gender are generally required to meet a weaker, but still stringent “exacting scrutiny” standard, that requires the government to show they serve an “important” government interest.

On the basis of the Equal Protection Clause—and the Fifth Amendment’s Due Process Clause, which has been interpreted as implicitly guaranteeing equal protection—the Supreme Court decided in Loving v. Virginia in 1967 that laws banning interracial marriage were unconstitutional. In his decision, Justice Earl Warren wrote that

Marriage is one of the "basic civil rights of man," fundamental to our very existence and survival.To deny this fundamental freedom on so unsupportable a basis as the racial classifications embodied in these statutes, classifications so directly subversive of the principle of equality at the heart of the Fourteenth Amendment, is surely to deprive all the State's citizens of liberty without due process of law. The Fourteenth Amendment requires that the freedom of choice to marry not be restricted by invidious racial discrimination. Under our Constitution, the freedom to marry, or not marry, a person of another race resides with the individual and cannot be infringed by the State.

To deny same-sex couples this fundamental right, defenders of same-sex marriage bans must show that denying people the right to marry others on the basis of their gender is not like denying them the right to marry others on the basis of their race, and that there is some compelling reason to do so.

While it’s true that many people think of marriage as fundamentally an institution involving a man and a woman—almost by definition—it’s hard to find legal reasons why that should be the case. The case against same-sex marriage generally involves bizarre and elaborate handwaving to cover a lack of coherent arguments. It generally depends on the idea that allowing gays to marry will—for some unspecified reason—weaken the institution of marriage for straight couples. And this, defenders argue, will—for some other vague and difficult to articulate reason—have all sorts of dire consequences for society and even human civilization.