A third federal challenge to restricting marriage to heterosexual couples opens Wednesday in Massachusetts.

Massachusetts Attorney General Martha Coakley filed the lawsuit last July on behalf of the 16,000 gay and lesbian couples who have legally wed in the state since gay marriage became legal in 2004.

US District Judge Joseph L. Tauro is expected to hear opening arguments in Commonwealth of Massachusetts v. US Department Health and Human Services today.

The lawsuit challenges section 3 of the Defense of Marriage Act (DOMA), the 1996 law that defines marriage as a heterosexual union for federal agencies. Section 3 denies gay and lesbian couples federal benefits such as federal income tax credits, employment and retirement benefits, health insurance coverage, and Social Security payments.

The lawsuit does not challenge section 2 of DOMA, which allows states to ignore legal gay marriages performed in other states.

“In enacting DOMA, Congress overstepped its authority, undermined states' efforts to recognize marriages between same-sex couples, and codified an animus towards gay and lesbian people,” the lawsuit says.

“[S]ame-sex couples in Massachusetts are still denied essential rights and protections because the federal Defense of Marriage Act interferes with the Commonwealth's authority to define and regulate marriage.”

The first federal trial to challenge the constitutionality of a gay marriage ban wrapped up in January after more than three week's worth of testimony. Lawyers representing a gay and a lesbian couple argued that California's gay marriage ban, Proposition 8, violates their constitutional rights. Closing arguments are expected to be heard in June.

A second federal challenge to DOMA has also been filed in Massachusetts by 17 gay men and lesbians who were married in the state. Judge Tauro is also presiding over that case.

Gay advocates believe all three cases will likely reach the Supreme Court.