WASHINGTON - The US Supreme Court seized center stage in a historic social policy debate over same-sex marriage on Friday by agreeing to review the validity under the US Constitution of a federal law defining marriage as a union between a man and a woman.

In an order, the court also announced that it would consider a challenge to California's ban on gay marriage, known as Proposition 8, which voters narrowly approved in 2008.

Same-sex marriage is a hot-button issue in a country where 31 of the 50 states have passed constitutional amendments banning it while Washington, DC, and nine states have legalized it, three of them on Election Day last month.

Yet even where it is legal, married same-sex couples do not qualify for a host of federal benefits because the 1996 Defense of Marriage Act, or DOMA, passed by Congress, only recognizes marriages between a man and a woman.

Gays and lesbians married under state laws have filed suits challenging their denial of such benefits as Social Security survivor payments and the right to file joint federal tax returns. They argue the provision, known as Section 3, violates equal protection provisions of the US Constitution.

Meeting in private on Friday at their last weekly conference before the court's holiday recess, the justices considered requests to review seven cases dealing with same-sex relationships. Five of them were challenges to the federal marriage law, one to California's gay marriage ban and another to an Arizona law against domestic partner benefits.

The court granted the appeal of Edith Windsor, an 83-year-old lesbian who challenged her denial of federal tax benefits under the Defense of Marriage Act.

The justices were widely expected to review at least one of the challenges to the federal marriage law, as two federal appeals courts had found the law unconstitutional.

Less expected was the court's decision to review California's ban on same-sex marriage. The California case, Hollingsworth v. Perry, had sought marriage equality for gays and lesbians under the US Constitution.

The 9th Circuit in February found the gay marriage ban unconstitutional, but it ruled narrowly in a way that only affected California and not the rest of the country, finding that the state could not take away the right to same-sex marriage after previously allowing it. No other state that allowed gay marriage has later banned it.

The court could follow the 9th Circuit's decision and also rule narrowly, allowing same-sex marriage only in California but not the rest of the country. Or it could recognize a right to marriage equality.