FACTBOX: U.S. laws on gay marriage, civil unions

(Reuters) - A federal challenge to a California gay marriage ban will go to trial in January next year, according to a schedule set on Wednesday by a U.S. judge.

Following is a look at laws on gay marriage and same-sex civil unions in the United States.

* Six states have approved gay marriage, although the action in Maine is being challenged.

* Forty U.S. states have laws explicitly prohibiting such marriages, including 29 with constitutional amendments restricting marriage to one man and one woman, according to the Human Rights Campaign, a gay rights advocacy group.

* The first legal same-sex marriages in the United States took place in Massachusetts in 2004, a year after the state's highest court ruled that a ban on gay marriage was unconstitutional. On July 8, Massachusetts sued the U.S. government to seek federal marriage benefits for same-sex couples in a move challenging the constitutionality of the federal 1996 Defense of Marriage Act. About 16,000 gay and lesbian couples have married in the state since 2004.

* California's Supreme Court backed a ban on gay marriage on May 26, upholding a voter-approved proposition defining marriage as between a man and a woman, but saying the marriages last year of 18,000 same-sex couples were still legal. Ted Olson and David Boies, the lawyers who faced off in the 2000 U.S. presidential election vote recount, have filed a federal lawsuit aimed at the Supreme Court arguing that a gay marriage ban is unconstitutional.

* Connecticut legalized gay marriage in October 2008, when its Supreme Court overturned a ban on same-sex marriage. Local authorities began issuing marriage licenses in November.

* Iowa's supreme court issued a unanimous ruling on April 3 that said a gay marriage ban violated the constitutional rights of gay and lesbian couples. The state's first legal same-sex weddings took place later that month.

* On April 7, Vermont lawmakers overrode a governor's veto of a gay-marriage bill, making the New England state the first in the country to legalize gay marriage with a legislative vote. Its law takes effect on September 1.

* The District of Columbia city council voted on May 5 to recognize same-sex marriages performed in states where those unions are legal, although gay marriages cannot be conducted in the city itself.

* Maine's governor signed a bill on May 6 that cleared the way for the legalization of gay marriage. The law is due to take effect in September. Opponents have gathered enough support for a statewide referendum on gay marriage, but the signatures have not been certified.

* The New York State Assembly passed a bill on May 12 to legalize gay marriage, but it faces an uncertain future in the state Senate, where a leadership struggle has deadlocked the body.

* New Hampshire on June 3 became the sixth U.S. state to authorize gay marriage after its Democratic-controlled House of Representatives endorsed gay marriage, hours after the state Senate approved the legislation along party lines. Governor John Lynch, a Democrat, signed the bill, which goes into effect on January 1.

* New Jersey permits same-sex civil unions that grant largely the same state rights as those afforded married couples -- from insurance coverage to tax benefits and hospital visiting rights -- but lack the full legal protections of marriage.

* Hawaii, California, Oregon and Washington offer gay couples some legal rights as partners.

* The patchwork of laws has caused some unusual complications. Rhode Island's top court, for example, ruled in December 2007 that a same-sex couple married in Massachusetts could not legally divorce in Rhode Island, saying the state's family court did not have authority over same-sex marriages.

* The U.S. Supreme Court has not taken a case on gay marriage, leaving states to decide on the issue, although the California federal challenge is aimed at eventually reaching the country's top court.

(Compiled by Jason Szep in Boston, Edith Honan in New York, Courtney Hoffman in San Francisco; editing by Todd Eastham)

(Washington World Desk: +1 202 898 8457)