The U.S. Supreme Court's decision in U.S. v. Windsor, striking down Section 3 of the Defense of Marriage Act, reflected themes that have been present in Justice Anthony Kennedy's jurisprudence for years: a concern about federalism and a desire to protect gays and lesbians from discrimination. These came together powerfully in a ruling that will matter for married same-sex couples all across the country. There are more than 1,100 federal laws that provide benefits to married couples. Now same-sex couples who are married in states that allow this will be treated the same under federal law as heterosexual couples.

The reasoning in Kennedy's ­majority opinion should be a surprise to no one who is familiar with his jurisprudence. He emphasized that the definition of who is married is traditionally left for state governments, raising federalism concerns about Sec­tion 3 of DOMA, which declares that for purposes of federal law and federal benefits marriage is between a man and a woman.

Since coming on the court in 1987, Kennedy has continually been a protector of states' rights. He consistently joined the conservative majority in limiting the scope of Congress' commerce clause power, restricting Congress' authority under Section 5 of the 14th Amendment, reviving the 10th Amendment as a limit on Congress' power and greatly expanding the scope of state sovereign immunity. But as much as Kennedy has been with the conservatives in federalism decisions, he has been with the liberal justices in advancing rights of gays and lesbians. In fact, he has written every major Supreme Court decision in American history doing so. In 1996, in Romer v. Evans, Kennedy wrote the opinion for the court striking down a Colorado initiative which repealed all laws in the state protecting gays and lesbians from discrimination and which precluded the enactment of any new such laws. Kennedy explained that the Colorado initiative was based on animus against gays and lesbians and that animus against a group is never a legitimate purpose sufficient to justify discrimination.

In 2003, Kennedy wrote the landmark opinion in Lawrence v. Texas, declaring unconstitutional a Texas law that prohibited "deviant sexual behavior," which was defined as sexual activity between a same-sex couple. Kennedy's opinion expressly overruled the earlier decision in Bowers v. Hardwick and concluded that if ­privacy means anything, it is what consenting adults do in their own bedroom.

In Windsor, Kennedy's opinion found that Congress' choice to refuse to recognize lawful same-sex marriages was based on animus against gays and lesbians and homosexual activity. As he did in Romer, Kennedy explained that such animus is insufficient to justify a law that discriminates on the basis of sexual orientation.

There is another similarity between his opinion in Windsor and his earlier ones in Romer and Lawrence: the Supreme Court invalidated the law without using heightened scrutiny for sexual-orientation discrimination. The U.S. Court of Appeals for the Second Circuit in striking down Section 3 of DOMA used heightened scrutiny. Lawyers challenging DOMA in the Supreme Court, including the solicitor general, urged the court to adopt heightened scrutiny for sexual orientation. But for Kennedy and the majority this was unnecessary. A law based on animus fails to meet even rational-basis review so there was no need to adopt a higher level of scrutiny.

LOOKING AHEAD

Of course, everyone will read Kennedy's opinion for a sign of how he and the majority will rule when they have to decide the constitutionality of state laws denying marriage equality to gays and lesbians. The court avoided that issue in dismissing Hollingsworth v. Perry, the case involving California's Proposition 8, on standing grounds. But the question is sure to be back before the court when laws in other states prohibiting same-sex marriage are challenged as violating the Constitution.

To be clear, Kennedy's opinion in Windsor is only about why Section 3 of the federal Defense of Marriage Act is unconstitutional. But the conclusion that it denies equal protection leaves no doubt that there are five votes on the current court to find a right to marriage equality for gays and lesbians. In fact, one must wonder whether Chief Justice John Roberts Jr. and Justice Antonin Scalia voted to deny standing in the Proposition 8 case for exactly that reason. If either had found standing, there would have been five votes for that, and the court would have had to reach the merits as to whether Proposition 8 is unconstitutional. In light of the five votes to strike down Section 3 of DOMA, they likely saw the same result as to Proposition 8 and thus a ruling that would have paved the way, if not required, that all states allow marriage equality.

In his vehement dissent in Windsor, Scalia said that there is no doubt that it is just a matter of time now before the majority will recognize such a right. On this he is right, and there seems little doubt that in the next few years it will happen and surely in an opinion by Ken­nedy. Perhaps his greatest legacy, and he undoubtedly knows this, will be as the justice in American history who did the most to protect the rights of gays and lesbians.

Erwin Chemerinsky is dean and distinguished professor of law, and Raymond Pryke Professor of First Amendment Law, at the University of California, Irvine School of Law.