The Supreme Court, striking at a time-honored principle of the insurance industry, ruled yesterday that employer retirement and pension plans may no longer calculate benefits differently for men and women.

The decision means that statistical tables showing that women live longer than men, which often cause retirement programs to make smaller payments to women each month, may not be used in employer-sponsored plans.

However accurate such tables may be, Justice Thurgood Marshall wrote for the court, they make sex distinctions, which are illegal under federal anti-discrimination laws. Sex, like race, cannot be a factor in employment, he said.

The 5-to-4 ruling, bitterly opposed by the insurance industry, was a victory for women's rights organizations. But much more is at stake in a similar unisex insurance battle being fought on Capitol Hill, where Congress is considering banning sex distinctions in all forms of insurance--whether or not provided by employers.

Yesterday's ruling applies to about 450,000 employer-sponsored retirement plans covering more than 26 million employes in private business and state and local government.

Many include annuity programs administered by private insurance companies that require equal contributions from women and men but pay women lower monthly benefits upon retirement in an effort to stretch out the money to cover the longer life span. That practice, at issue in yesterday's case, is now illegal.

In a separate 5-to-4 vote, controlled by a different majority of justices, the court said its ruling is not retroactive. The plans thus do not have to reimburse current retirees for past losses due to discrimination or go back and readjust past contributions for current workers. Justice Lewis F. Powell Jr., writing for the court on that issue, said retroactivity might bankrupt companies.

The decision applies only to future contributions and benefits. It does not mean women will necessarily get more money. All that is demanded is equalization.

The estimated cost of equalizing such programs ranges from $85 million to $675 million per year, depending on how it is accomplished, according to Department of Labor figures cited by dissenting justices yesterday. Employers and insurance companies can also drop the annuity option if they choose.

At issue in yesterday's case were sex-based mortality tables, used by the entire industry, which show that women tend to live about seven years longer than men. In private life insurance policies, that generally means women pay lower premiums for the same benefits. In annuities, however, the opposite is true.

Nathalie Norris, an Arizona state employe, signed up for an annuity plan offered by the state through the Lincoln National Life Insurance Co. in 1975. The annuity plan was one of several options available to Arizona employes under a deferred compensation program. A portion of Norris' paycheck was channeled tax-free into investment to be returned upon retirement, at which time taxes would be due.

Norris' monthly contribution was $199, the same as from any male employe earning the same salary. At retirement, however, she would receive $34 less each month because she is a woman. Over 20 years, the difference would be about $8,100.

She challenged the practice in a class action under Title VII of the Civil Rights Act of 1964, which bars sex, race and ethnic discrimination in employment. She cited a 1978 Supreme Court decision, Los Angeles Department of Water and Power vs. Manhart, in support of her claim. The Manhart decision prohibited an employer from requiring women to make larger contributions in order to obtain the same monthly benefits as men.

The same prohibition, Marshall said yesterday, applies to the unequal benefits. The law "requires employers to treat their employes as individuals not 'as simply components of a racial, religious, sexual, or national class.' Manhart squarely rejected the notion that, because women as a class live longer than men, an employer may adopt a retirement plan that treats every individual woman less favorably than every individual man."

It is true, Marshall said, that actuarial studies identify differences in life expectancy based on sex. But they also show differences based on race or national origin.

If the insurance industry's interpretation of the law were correct, he said, such actuarial studies "could be used as a justification for paying employes of one race lower monthly benefits than employes of another race . . . . If it would be unlawful to use race-based actuarial tables, it must also be unlawful to use sex-based tables . . . ."

Marshall also rejected the State of Arizona's contention that the insurance company, not the employer, is responsible for any discrimination. That would have placed the practice beyond challenge under the Civil Rights Act, which applies only to employers.

Justices William J. Brennan Jr., Byron R. White, John Paul Stevens and Sandra Day O'Connor joined Marshall in upholding the 9th U.S. Circuit Court of Appeals.

Justice Lewis F. Powell Jr., joined by Chief Justice Warren E. Burger and Justices Harry A. Blackmun and William H. Rehnquist, dissented. Powell said the discrimination law was not intended to cover either the insurance industry or actuarial tables. "Insurance companies cannot make individual determinations of life expectancy," Powell wrote. "They must consider instead the life expectancy of identifiable groups . . . ."

He said, "Sex-based mortality tables reflect objective actuarial experience. Because their use does not entail discrimination in any normal understanding of that term, a court should hesitate to invalidate this long-approved practice on the basis of its own policy judgment."

Powell said the holding in Arizona Governing Committee for Tax Deferred Annuity and Deferred Compensation Plans, et al, vs. Nathalie Norris etc. "will have a far-reaching effect on the operation of insurance and pension plans . . . . to the probable detriment of all employes."

On the retroactivity issue, O'Connor joined Powell and those dissenting on the merits of the case, creating a majority against retroactive application.