Some things are so bad that almost no reason will justify them. Today, segregating people by race is one of those. Laws or regulations that do this are subject to "strict scrutiny," which is shorthand for "explain yourself right now, buster." The only way such a law can pass is if the government can show that (1) it's trying to do something of huge importance and (2) there's really no other way to do it. Some government measures pass strict scrutiny -- but for most, it is considered "strict in theory, fatal in fact."

Most things government does, though, are just ordinary. Driver's license laws are a slap in the face to 15-year-olds; zoning laws mean that some property owners can't strip-mine their lots. For most laws, the "scrutiny" is "rational basis." If there might be a halfway decent reason, then the measure passes "rational basis" review. For most purposes, laws fail this test only when the reason behind them is "animus" -- "we don't like your kind." This test might be paraphrased as, "Is this law crazy or just plain mean? If not, okay."

For everyone who believes law is just partisan politics: Chief Judge Dennis Jacobs, whose majority opinion breaks new ground for gay rights, was appointed by President George H.W. Bush.

There's a gray zone between strict and rational basis. Sex discrimination, for example, is subject to "heightened" scrutiny -- not quite "strict," but pretty darn stern. Until now, the Supreme Court and the Courts of Appeals judged discrimination against gays by asking only, "Is this crazy or mean?" The Supreme Court has held that Colorado's anti-gay-rights initiative and Texas's anti-gay-sex law were motivated by "animus," lawyer talk for "we don't like your kind." But the court has never said that discrimination against gays requires "heightened scrutiny."

DOMA doesn't say that same-sex couples can't marry. If state governments allow them to marry, it says, fine; but other states don't have to recognize that marriage, and the federal government can't. For federal tax and benefit purposes, among others, "the word 'marriage' means only a legal union between one man and one woman as husband and wife."

Edith Windsor and Thea Clara Spyer were a committed couple for more than 40 years, and "domestic partners" under New York law since 1993. But like many same-sex couples, they craved marriage, and thanks to Canadian law, they had it for two years before Spyer died in 2009.

Then came the federal tax bill to Windsor for Spyer's estate: $363,053 higher than an opposite-sex married couple would have paid, thanks to DOMA. Windsor sued for a refund.

The District Court struck down DOMA without deciding a level of "scrutiny." That's because it said the law was animated by the "desire to harm a politically unpopular group" (in other words, "just plain mean"), and thus failed any level of review.