FACTBOX-Gay marriage laws in the United States

Dec 6 (Reuters) - Federal appellate judges in San Francisco hear arguments on Monday on whether California's gay marriage ban violates the U.S. Constitution.

A lower court said the ban was invalid but that ruling is on hold pending appeal.

The 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 top U.S. court.

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

* Five of the 50 states and the District of Columbia allow same-sex marriage: Iowa, Connecticut, Massachusetts, New Hampshire and Vermont.

* Forty-one states explicitly prohibit such marriages, according to Human Rights Campaign, a gay rights advocacy group.

* 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.

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

* 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. That amendment is the basis of the case under review in San Francisco. (Compiled by Ros Krasny in Boston, Edith Honan in New York, Peter Henderson in San Francisco and Dan Whitcomb in Los Angeles; Editing by John O'Callaghan)