Seeking to claim one of the key benefits of marriage, five same-sex, bi-national couples have filed a lawsuit aiming to overturn the U.S. federal ban on same-sex marriage.

Even in the few states where same-sex marriage is legal, the federal Defense of Marriage Act prevents married same-sex couples from receiving the more than 1,000 federal entitlements that would otherwise flow from marriage. Among those is an American citizen's ability to sponsor an immigrant spouse for legal residence.

Americans who are married to immigrants of the same sex do not have that right, putting same-sex binational couples at risk of either being separated by deportation or being forced to leave the United States in order to stay together.

The lawsuit, filed Monday in the United States District Court for the Eastern District of New York, challenges that situation.

For immigration purposes, whether the federal government recognizes a couple's marriage can determine whether a family may remain in the United States and live together, or may be torn apart, the complaint reads.

The complaint, which was filed on behalf of the five couples by the gay advocacy organization Immigration Equality, argues that the Defense of Marriage Act contradicts other immigration laws. The United States visa system is designed to emphasize family unity -- there is no annual limit on the numbers of visas for immigrants married to American citizens, for instance -- so the Defense of Marriage's lack of recognition of gay marriage upends that normal operation of the law, the complaint's authors wrote.

The federal government also has set the preservation of families as a national priority in the context of immigration law, the authors wrote, but because of the Defense of Marriage Act, it is the federal government that threatens to tear binational families apart.

More fundamentally, the Defense of Marriage Act violates the Constitution by mandating different legal treatment for gay couples, said Victoria Neilson, the legal director for Immigration Equality and a co-counsel in the case.

The legal argument is a straightforward equal protection argument, Neilson said. There are married couples in the states of New York, Connecticut and Vermont who can sponsor their spouses for green cards, and the only thing preventing the couples in our case from doing that is the fact they are in lesbian and gay relationships. The Constitution does not allow that kind of discriminatory treatment between two groups that are identical.

Those three states have all legalized same-sex marriage.

Obama Opposed to DOMA

President Barack Obama has signaled his opposition to the Defense of Marriage Act by ordering the Justice Department to no longer defend the law in court. But the president has said the onus for changing the law falls on Congress, a position that is consistent with his tactic of expressing cautious support for some recognition of same-sex unions without directly jumping into the legal battles surrounding the issue.

It's not unprecedented for immigrants in same-sex couples to be granted reprieves from imminent deportation, and advocates are hopefully watching a new policy that directs immigration officials to set aside low-priority deportation cases. But more than the threat of deportation, Neilson said, the Defense of Marriage Act injects uncertainty into the lives of same-sex couples.

A married couple represented in the lawsuit, for example, is eager to have children. But one of the women in the couple, Maria del Mar Verdugo, was born in Spain, so they put their plans on hold.

Not knowing whether Mar can stay in the U.S. permanently, that makes it very difficult to plan a life together, Neilson said.