BOSTON — Nancy Gill has worked for the Postal Service for almost 23 years. But because she is married to a woman, she cannot provide the same health benefits to her spouse that her co-workers at the post office can provide for their families.

Ms. Gill, 51, and Marcelle Letourneau, 47, married in Massachusetts in 2004 and are the lead plaintiffs in a suit challenging the federal law — the Defense of Marriage Act, known as DOMA — that defines marriage as being between a man and a woman. The women, who live in Bridgewater, Mass., are challenging the section that denies marriage-related benefits to same-sex couples, saying they are being denied equal protection under the law.

The case, filed in March 2009, was argued Thursday in Federal District Court here before Judge Joseph L. Tauro. It is the first major challenge to the act and is likely to end up before the Supreme Court.

Mary L. Bonauto, director of the civil rights project for Gay and Lesbian Advocates and Defenders, argued on behalf of the couple and 15 other plaintiffs, calling the case “a classic equal-protection issue.”