The Reagan administration may be moving toward a new defense of the domestic spending cuts that have embroiled the president in the "fairness issue."

It revolves around the fact that so many of the nation's poor--especially poor blacks--are members of female-headed families.

The argument is that the plight of such people, deriving as it does from family structure, may be beyond the reach of government. Sometimes it is also argued that government makes the problem worse.

About a seventh of U.S. families are now headed by women; among blacks the proportion is closer to half. And roughly half the families below the government poverty line are in this category.

Clarence Thomas, head of the Equal Employment Opportunity Commission, said at the Urban League convention earlier this month that "family composition . . . can have as much impact on employment as traditional barriers caused by discrimination."

Morris Abram, the senior of Reagan's three new nominees to the U.S. Commission on Civil Rights, said in an interview recently that a "permanent underclass" is dragging down black income figures and that "quotas, even if you think they are a good idea, cannot solve a sociological problem."

Bruce Chapman, the former head of the Census Bureau who now studies demographic trends for the White House, said, "The recovery will have to be sustained and strong for years before it will cut into poverty among female-headed households."

Robert B. Carleson, special assistant to the president for policy and a long-time Reagan adviser on welfare, remarked that "The increase in female-headed households is not a result of government policy, and I'm not sure it's reasonable to ask government policies to bring families back together . . . . There is no clear solution in sight."

And Reagan, reaching back to a theme that he also used in the 1980 campaign, said in a speech two weeks ago in Atlanta to the American Bar Association, "Let us have the courage to speak the truth. Policies that increase dependency and break up families are not progressive, they're reactionary, even though they are invariably promoted, passed and carried out in the name of fairness, generosity and compassion."

"It's not a rationale, it's an inescapable observation," said a senior administration official after Census reported last week that the national poverty rate rose to 15 percent and the black rate to 35.6 percent in 1982, partly because of the recession and partly because of budget cuts.

"There is no doubt of what progress black Americans have made in income relative to whites . . . . The problem is a social or cultural one that is hurting the black community."

But the administration's family-structure and dependency arguments have a double edge; administration opponents say the increase in female-headed families shows the need for more welfare spending, not less.

(They also make the point that two-thirds of the women who were heads of households held jobs the last time the government counted and that welfare households average fewer than three years on the rolls. This, they say, gives the lie to the idea that there is a permanent welfare subculture in our midst; they argue that the welfare population is a floating one.)

In May the Civil Rights Commission issued a report entitled "A Growing Crisis: Disadvantaged Women and Their Children" suggesting that discrimination has helped perpetuate poverty.

"Today women who maintain their own families are an ever-increasing proportion of the poverty ranks," the report said, and "in general women have been restricted to low-paying jobs in occupations with limited potential for advancement."

Democrats and civil rights leaders still have faith in government intervention. Eleanor Holmes Norton, head of the EEOC in the Carter administration, says "there can be success in dealing with female-headed households if the government is willing to invest real resources in the short-term . . . . Single mothers have very high levels of motivation.

"They have a child to work for. There has been growth in female headed households but no change in government policy. That is inexcusable . . . . Job training and affirmative action for women, especically black women, is well within the scope of government."

Thomas Joe, head of a research group that recently studied the economic status of blacks, also looks to job-training programs as an answer.

And he adds that "Of the 12 million on welfare, 8 million are kids. We can't go on punishing the kids for the problems of their parents. Half of the black families are female-headed and 76 percent of black children from female-headed families live in poverty. What chance do they have of living a better life?

"The Reagan administration may be right that it can't do much to keep people married," says Joe. But he argues that "with strong affirmative action for women and a strong commitment to equal rights for women in hiring, they can help women and their children to pull themselves out of poverty."