Factbox: Gay marriage in the United States

ALBANY (Reuters) - New York became the sixth U.S. state to allow gay marriage on Friday. The state-by-state battle over gay marriage has become a contentious U.S. social issue ahead of the 2012 presidential and congressional elections.

Following is a look at laws on gay marriage in the United States:

Six of the 50 U.S. states and the District of Columbia now allow same-sex marriage: Iowa, Connecticut, Massachusetts, New Hampshire, Vermont and New York, the third most populous state in the country.

Four states Delaware, Hawaii, Illinois and New Jersey have approved gay civil unions. But gay marriage is specifically banned in 39 states.

In California a judge last year overturned a ban on gay marriage, but no weddings can take place while the decision is being appealed. It could set national policy if the case reaches the U.S. Supreme Court.

Four states -- New Jersey, New Mexico, New York and Rhode Island -- do not explicitly prohibit gay marriage but have not endorsed it, according to DOMA Watch, an advocacy group that supports limiting marriage to men and women.

Courts and state legislatures have legalized gay marriage in the United States but popular votes have consistently opposed same-sex unions, most recently in Maine.

Arizona is the only state where voters rejected a constitutional ban on gay marriage, in 2006, but they approved a similar measure in 2008.

California's top state court in 2008 ruled that a ban on same-sex marriage violated the state constitution. But a few months later, voters amended California's constitution, defining marriage as between a man and a woman.

The first legal same-sex marriages in the United States took place in Massachusetts in 2004.