By Brendan O'Brien
MILWAUKEE (Reuters) - Four same-sex couples on Monday asked a federal judge in Wisconsin to overturn the state's constitutional ban on gay marriage, the latest in a series of court challenges aimed at making same-sex marriage legal around the United States.
The couples suing Wisconsin said an amendment to the state constitution banning gay marriage and civil unions, approved by Wisconsin voters in 2006, unconstitutionally blocks them from federal protections afforded couples in other states.
The lawsuit follows high-profile U.S. Supreme Court rulings on gay marriage last year that paved the way for gay marriage to resume in California and struck down the Defense of Marriage Act, a federal law that denied federal benefits to legally married same-sex couples.
The Supreme Court's ruling on the Defense of Marriage Act was cited by federal judges overturning bans in Utah and Oklahoma, whose rulings are on appeal. Challenges to bans also are pending in Michigan, Virginia and Florida.
In Wisconsin, the American Civil Liberties Union is representing the four couples who are challenging the state ban in a lawsuit filed in U.S. District Court in Madison.
"The state's constitutional marriage ban sends a message that lesbians, gay men and their children are viewed as second-class citizens," the ACLU said in a statement.
The lawsuit names Wisconsin Governor Scott Walker, a Republican, and other state and local officials as defendants in their official capacity. A representative of the governor could not be reached immediately for comment.
The suit also said Wisconsin statute "discourages" same-sex couples from marrying in another state, referring to a Wisconsin law that threatens nine months in jail and a $10,000 fine for couples that do.
Barely a decade ago, no U.S. states recognized same-sex marriage. Seventeen states plus the District of Columbia now recognize same-sex marriage, including eight states where it became legal in 2013.
Same-sex marriage is legal in neighboring Minnesota and Iowa and legislation making gay marriage legal in Illinois takes effect on June 1, though some marriages have been performed in Illinois on an emergency basis.
Thirty-three states ban gay couples from marrying by state constitutional amendment, statute or both. In Indiana, where gay marriage is banned by statute, lawmakers are considering putting a proposed state constitutional amendment banning same-sex marriage before voters in November.
(Editing by David Bailey and Mohammad Zargham)