It's been called everything from a "sin subsidy" to a "divorce bonus" to a "marriage penalty." But by any name -- "even the unprintable ones," says Washington postman Roscoe Barnes, "the tax structure still seems just plain unfair."

The Barneses are one of an estimated 20 million couples affected by the so-called "marriage tax" -- which results in married couples with two incomes paying more tax than they would if they were single. With a combined income of roughly $39,000, the Barneses will pay about $1,400 more income tax than two single people with the same income.

"It's such a contradiction that a government says it's against immorality, yet penalizes you for being married," he says. "I've known people who wanted to get married, but lived together because it was cheaper.

"My wife and I discussed that, but we really wanted to be married in spite of the tax. I call it the price of love."

This "love price" wasn't always so dear."The tax system was devised years ago when most families had just one wage earner," says an aide to Sen. Charles Mathias (R-Md.), who has introduced a bill that would allow married couples the option of filing separately using the rate schedule for single people.

"At that time married couples who filed jointly had a great advantage over single people. To remedy that, in 1969 Congress enacted a special rate schedule for singles. This created a 'marriage tax penalty' when single people with two incomes married."

The "penalty" has two sources. One is that, in effect, the second earner is taxed at a higher rate -- since the first dollar of the second earner's income is taxed at the same rate as the last dollar made by the first earner. Also, married couples filing jointly are permitted a $3,400 zero bracket amount (standard deduction), while singles get more than half that amount, $2,300. (A married person filing separately is permitted $1,700.)

Back when most families had just one earner, the "marriage tax" wasn't viewed as a large problem. But as increasing numbers of women join the workforce, more and more couples are affected.

"More than half of all married women in the country are working," notes Rep. Millicent Fenwick (R-N.J.), who has introduced a bill to eliminate the "marriage tax" each year since 1975, and plans to reintroduce it today.

"This means that more than half the nation's married taxpayers may be paying the 'marriage tax.' And experts estimate that anywhere from two-thirds to three-quarters of all married women under age 55 will be working by the end of the decade."

The biggest problem with the "tax on marriage," says Rep. Fenwick, "is that it encourages divorce and provides an incentive not to marry. I get letters from women who say 'Hurry up and pass the bill. My husband's nagging me to get divorced to cut our taxes.'

"Marriages are already under considerable strain. The tax is socially damaging. It's also a disincentive to women going back to work. One woman wrote me that it costs her $15 a day in extra tax for her to be working."

Public awareness of the problem has also increased, fanned by press reports about couples like David and Angela Boyter, who have divorced three times and remarried twice to avoid paying the "marriage penalty."

Support of its elimination is widespread. Both the 1980 Republican and Democratic Party platforms called for equity in tax treatment of married couples. A Gallup poll reports that 83 percent of Americans favored elimination of the "marriage tax."

Although at last year's White House Conference on Families, delegates were sharply divided on many topics, "No issue drew more support than the proposal to eliminate the marriage tax," said conference executive director John L. Carr.

"In our hearings we heard from individuals who had postponed or avoided marriage because of the financial consequences. We also heard from people whose disbelief had turned to anger at the realization that they pay more taxes than the couple down the street because they were married to the person they live with.

"Our tax code probably reflects our values more accurately than our rhetoric on Mother's Day or the Fourth of July. . . . Those are dollars that can pay for child care, shoes, college tuition, dental bills or even a long overdue vacation."

One reason that bills like Fenwick's and Mathias' have succeeded in the past is that the "marriage tax" contributes an estimated $10 billion a year to federal coffers.

But with the Reagan administration's professed interest in tax cuts, Rep. Fenwick says she thinks the time is ripe for passage. "It's the most popular bill in Congress," she claims, noting that she's discussed her bill with Reagan, and "he likes it."

Two-income couples in the Washington area also have expressed their approval of the bill.

"I'm not big on tax shelters or dodges," says Washington attorney Frederick Dorsey, "because I believe in paying my share. But I think it should be my fair share. I see it as a type of discrimination against married people."

Dorsey and his wife, Gail Gerebenics, had planned to marry in September of 1979, but postponed their wedding until January of 1980 when they discovered it would save them roughly $3,500 in additional taxes.

"I think it's an outrage," says Gerebenics, who is also an attorney and together with her husband now makes about $90,000. "Everything is so expensive these days that our standard of living is not as high as you might think.

"It's easy to say that a couple can avoid the marriage tax if only one of them works. But what family these days can live on one salary?"

"My lawyer friends tell me to get a divorce," says Mary P. Hartzler, 49, who estimates that she and her husband pay about $2,400 in "marriage tax" on their combined income of $50,000.

"But in Virginia it's illegal to live together. Plus, we'll be celebrating our 31st anniversary, and our commitment means a lot to us. We think it's very unfair that the government taxes us higher because we're married."

While none of the couples interviewed said they would divorce and remarry to avoid paying the "marriage penalty," one man summed up his feeling about those people who do: "I don't think it's any worse than rich people finding shelters to avoid their taxes. To me they're working-class heroes." CAPTION: Chart, Chart your own "marriage tax." Examples are for couples with no dependents and no itemized deductions. Boxed area shows "tax" for combined income of $30,000. (For the Washington area, the median in 1980 was $27,000.)