When the city of Cambridge issues paychecks to its public employees, nearly two dozen workers find a federal tax on their income that their colleagues don't have to pay.

Like many people, these 22 school and city workers chose to put their spouses on their employer-provided health insurance. Because they're in a homosexual relationship, the value of that health coverage is considered taxable income by the federal government.

But starting this month, Cambridge will become what is believed to be the first municipality in the country to pay its public employees a stipend in an attempt to defray the cost of the federal tax on health benefits for their same-sex spouses.

The city employees hit by the extra tax pay an additional $1,500 to $3,000 in taxes a year and officials estimate the stipends would cost the city an additional $33,000.

"This is about equality," said Marjorie Decker, a Cambridge city councilor. "This is a city that models what equality really means."

Of the thousands of legally married gay and lesbian couples in Massachusetts, none can receive the federal benefits offered to heterosexual married couples because the federal government doesn't recognize same-sex marriages.

Those benefits include Social Security survivors' benefits, immigration rights, family leave and the ability to file joint tax returns.

The council last month approved the measure that would provide quarterly stipends to any city or school employee who puts a same-sex spouse on their health insurance. The vote came after council members began looking in January for a way to offset what they called an unfair and discriminatory tax.

"This is ultimately a fairness issue. Two people who do the exact same job should be paid exactly the same for what they are doing at work," said Leland Cheung, a Cambridge City Councilor who pushed for a proposal with fellow councilor E. Denise Simmons, who is openly gay.

Decker and Cheung said the additional funds needed from the city's personnel budget is a minor cost in the city's more than $500 million budget, but some say the public's money should not be used to go against established law.

"It's a travesty of using taxpayer monies to circumvent a national policy," said Kris Mineau, president of the Massachusetts Family Institute, an advocacy group opposed to same-sex marriage.

The 15-year-old federal Defense of Marriage Act defines marriage as being between a man and a woman, allows states to deny recognition of same-sex unions performed elsewhere and prevents the federal government from recognizing gay marriages. This provision federal prevents even gay and lesbian couples married legally by a state 7/8-- as in Massachusetts -- from receiving any federal benefits.

The act has faced setbacks lately, including President Obama's order in February for the Justice Department to stop defending the constitutionality of the federal law.

The key part of the law denying federal benefits for married gay couples was also ruled unconstitutional in the state by a U.S. District judge in Boston last year. That decision is being appealed.

Mary Bowe-Shulman of Acton, Mass., who was one of the plaintiffs in the case against DOMA, said she was happy that Cambridge employees would see financial relief and that the city was taking action on what she saw as a civil rights issue.

"I think that's a wonderful thing for them to make up for that," she said.

Bowe-Shulman, a staff attorney at the Massachusetts Court of Appeals, said she loses $7,800 a year to federal taxes on health insurance for her wife of 6 years, money she would rather be putting in a college fund for the couple's two children.

"It just makes me feel like my family is being treated differently than everyone else's," she said.

Although Bowe-Shulman applauded the Cambridge policy, she said hopes the court ruling stands so that such policies are unnecessary.

"I think it's just ridiculous that the state would have to expend some of its limited dollars to make up for this discriminatory policy," she said.

While Cambridge is known for its liberal policies and has had three consecutive openly gay mayors, the city is not the only employer offering additional pay.

Since 2009, at least 17 companies that offer benefits to same-sex spouses or domestic partners have supplemented employees' income to cover the tax, according to Sarah Warbelow, a legislative director for the Human Rights Campaign. Some of those companies include Mountain View, Calif.-based Google Inc., Palo Alto, Calif.-based Facebook and the Boston Consulting Group.

The number of same-sex spouses and partners receiving health benefits will soon increase as New York became the sixth and largest state to legalize gay marriage, joining Iowa, Connecticut, New Hampshire, Vermont and Massachusetts. The District of Columbia also recognizes gay marriage.

Thirty states have constitutional amendments banning gay marriage.

New legislation is being proposed nationally by Sens. Charles Schumer, D-N.Y., and Susan Collins, R-Maine, to eliminate the tax on health insurance for same-sex partners and spouses.

The lawmakers say this change will help individuals and reduce payroll taxes of the employers that offer benefits to same-sex partners and spouses without forcing employers to recognize domestic partners. Opponents say the proposal conflicts with current law.

"This appears to be another tactic to circumvent the intent of the DOMA, which is that federal tax dollars will not be used to benefit any kind of relationship outside a marriage between a man and a woman," said Mineau, who also criticized Obama's refusal to defend DOMA.

Still, Cambridge lawmakers are optimistic the policy will spread to other municipalities and states."Cambridge is a city on the cutting edge, and I think this is another example of that," Cheung said.