Edith Windsor sued to get her tax refund.She and Thea Spyer, who lived in New York state, had been together for more than 40 years when they got married in Canada in 2007. Spyer died two years later and left Windsor a considerable estate.Windsor paid federal estate taxes of $363,000 then sought a refund under a law that generally allows a spouse to inherit an estate without getting hit by estate taxes.But the Internal Revenue Service rejected Windsor's request.Under the 1996 Defense Of Marriage Act, Windsor and Spyer weren't considered spouses. They're both women, and DOMA doesn't recognize same-sex marriages for purposes of any federal law -- and that means more than 1,100 of them, such as those governing Social Security or veterans' benefits.So Windsor sued, arguing that federal law can't discriminate in this fashion, and her case is one of the ways the issue of gay marriage and equal protection under the Constitution reached the U.S. Supreme Court.The justices have agreed to hear arguments, probably in March, over DOMA (U.S. v. Windsor, 12-307) and in a separate case challenging California's Proposition 8, which added a same-sex marriage ban to the state Constitution (Hollingsworth v. Perry, 12-144).But only when decisions are issued, most likely in June, will it be certain whether the court deals broadly with the constitutionality of same-sex marriage or rules more narrowerly.There are many tensions raised by these cases: States' rights : Marriage typically has been regulated by the states, and legal compacts from one state have been given full faith and credit in the others. DOMA intrudes into all that. While DOMA says states still can decide how to define marriage, the federal law changes the equation in numerous ways and prevents gay marriage contracts in one state from being honored in another.Texas, exercising its state rights can refuse to honor a marriage license granted by Maryland in exercise of its state rights. Can Congress step in this way without overstepping its power? Equal protection : California voters approved Proposition 8 in 2008. But it revoked same-sex marriage rights that the California Supreme Court said the state Constitution protected. The 9th U.S. Circuit Court of Appeals ruled that taking away those rights violated equal protection under the U.S. Constitution. When it comes to fundamental rights, states can't treat residents differently without an appropriate reason. Does everyone have a basic right to marry? Do states have a good enough reason to tell some of them they can't? Politics and law : The U.S. Justice Department has stopped defending DOMA but urged the justices to take up the Windsor case or one of several others raising similar issues. Because of the administration's position, the three Republican members of the Bipartisan Legal Advisory Group in the U.S. House has intervened, but it's not clear whether they have any authority to defend the law on behalf of Congress.California Gov. Jerry Brown and the state attorney general aren't defending Prop 8, either, so supporters of the initiative are trying to get it reinstated. But a leader of the legal team opposing Prop 8 is Theodore Olson, a GOP stalwart who was President George W. Bush's solicitor general, was an official in President Ronald Reagan's Justice Department and won the Bush v. Gore cases stemming from the 2000 presidential election and the Citizens United case involving campaign finance restrictions. The cultural divide : The Pew Forum on Religion & Public Life reports that popular opinion on same-sex marriage has shifted notably and in a relatively short timespan: In 2001, Americans polled opposed same-sex marriage, 57 percent to 35 percent; but today, 48 percent favor it, while 43 percent are opposed. (bit.ly/U0tSH5)While 30 states, including Texas, have constitutional language banning same-sex marriage, Maine, Maryland and Washington state recently legalized it, making 10 jurisdictions (including Washington, D.C.) where gay people can marry. Ten states recognize civil unions or domestic partnerships -- including California, which currently stops just short of marriage.Sometimes, the Supreme Court follows public opinion, sometimes the justices lead it. Where the court comes down on these cases almost surely will help shape already-changing public opinion.