FRAMINGHAM, Mass. – Until tax season comes around, Mary Ritchie and Kathy Bush can act like any legally married couple raising two boys. In midwinter, that means watching lots of basketball.

“We go to every game and every practice,” Bush said.

But when they file their federal tax return, they are no longer married. The return is supposed to be “true, correct and complete” by law, but they cannot check “married, filing jointly.” The loss of this routine break costs them up to $6,000 a year.

Ritchie, a state police detective, and Bush, who worked at the New England Journal of Medicine before the boys were born, were wed in 2004, soon after Massachusetts became the first state to allow gay marriage. And four years ago, they joined the first broad lawsuit challenging the federal Defense of Marriage Act, or DOMA, of 1996. They say the law violates the Constitution’s promise of equal protection of the laws.

Now the issue is before the Supreme Court, one of two gay marriage controversies to be heard this month. Both raise questions of equality and fairness. On March 26, the court will consider California’s Proposition 8 to decide whether the voters’ ban on same-sex marriage wrongly denies gays and lesbians an equal right to marry. And on March 27, the court will consider whether DOMA wrongly denies married gay couples equal benefits under federal law.

Legal experts on the left and right foresee the DOMA case yielding a landmark ruling. “It’s enormously important. No gay person is really married in this country as long as DOMA remains on the books,” said Walter Dellinger, a former Clinton administration lawyer who says the law should be struck down.

Nine states, including New York and nearly all of New England, allow gay marriage. The nation has more than 114,000 married same-sex couples, according to the Williams Institute at UCLA. And one-fifth of the couples are raising children. Federal law has more than 1,000 benefits that turn on marital status, including lower taxes and Social Security for surviving spouses. Members of the military and federal employees cannot include a same-sex spouse on their health plan.

Social conservatives urged the court to reject the claim that gays are a minority deserving of special protection. But if the court treats gays as victims of discrimination, “it will send a signal there is a constitutional right to marriage,” said Paul Linton, counsel for the Thomas More Society.

Though it is less than 20 years old, DOMA is seen by some as outdated, given the rapid changes in laws and in public opinion on gays and lesbians. “It was a very different time,” said former President Bill Clinton, explaining recently why he thinks the law he signed should be struck down.

In 1996, same-sex marriage was legal nowhere in the country, but the Hawaii Supreme Court had hinted it might allow such marriages. In Congress, lawmakers worried that if gay marriage became law in one state, all states would be forced to honor such marriages through the Constitution’s “full faith and credit” clause, which requires states to abide by court judgments.

To head off this prospect, the law said no state need recognize a same-sex marriage. Almost as an afterthought, it also said the federal government would not recognize them either.

The challenge to DOMA began in Boston with Mary Bonauto, a lawyer for Gay & Lesbian Advocates & Defenders. She had won the original gay marriage ruling from the Massachusetts high court in 2003, and she believed seeing gay couples marry caused many to rethink their reflexive opposition to the idea.

“There was an outbreak of happiness. Weddings are joyous occasions, and they helped transform the debate,” she said.

But practical problems remained because of DOMA.

Nancy Gill, a postal worker, could not put her spouse on her health insurance plan. She became the lead plaintiff in the suit. Many others, like Ritchie and Bush, faced tax problems. Still others faced the hardship of a spouse dying after a long illness. Herb Burtis, 81, spent 16 years caring for his partner of 60 years, but he was denied the survivors benefit under Social Security that goes to other married survivors.

Bonauto saw this as fundamentally unfair. “In our nation’s history, the government (in Washington) has respected a state’s determination of who is married. But this law singles out one category for marriages for public disrespect,” she said.

The Obama Justice Department initially opted to defend DOMA, but in July 2010, U.S. District Judge Joseph Tauro, a President Richard Nixon appointee, ruled it unconstitutional. It violated the rights of legally married gay couples, he said, and intruded on the state’s turf in deciding who is married.

The administration appealed, but soon had second thoughts. In February 2011, President Barack Obama and Attorney General Eric H. Holder Jr. announced that legal discrimination against gays and lesbians could not be justified, they said.

In their place, House Republicans moved to defend DOMA and hired former Solicitor Gen. Paul Clement to argue the case. But last year, the U.S. court of appeals in Boston also said the law was unconstitutional.

Both sides agreed to send the Gill case to the Supreme Court for a final decision, but there was a glitch. Justice Elena Kagan had been Obama’s solicitor general when the administration first defended the law, and she said she would not take part in deciding a case that she had worked on. Kagan specifically mentioned the Gill case.

So, the Supreme Court announced that all nine justices would decide a case filed later by a New York woman, Edith Windsor. She was required to pay $363,000 in estate taxes after the death of her spouse, Thea Spyer. A surviving spouse would not owe any such tax if the government recognized the marriage as legal. The case from Massachusetts, joined by the two women from Framingham, will be held by the high court and resolved based on the ruling in the New York case.

Reflecting on the last two decades, Ritchie and Bush say they have been surprised at how much has changed for lesbians and gays. “We’ve been very fortunate,” Bush said, because they were accepted in the community as just another married couple with children.

If they win at the Supreme Court, the lawsuit could yield an asset for their sons’ future. The judge’s 2010 decision tallied year by year the amount that Ritchie and Bush had overpaid their federal taxes because they were barred from filing as a married couple. The total after nine years is about $40,000.

“It would be nice to have that for the boys’ college education,” Bush said.