American women - as a class - live about five years longer than American men - also as a class. Does an employer engage in illegal sex discrimination by charging an induividual woman more than an individual man for identical monthly pension benefits?

No, the Supreme Court was told yesterday by Los Angeles Deputy City Attorney David J. Oliphant, who said that acturial table show that women, because they outlive men, will on the average, draw larger total sums in pension benefits than men.

Yes, the justices were told by Robert M. Dohrmann, attorney for women who have sued the Los Angeles Department of Water and Power for refunds of "excess" deductions from their paychecks under a mandatory retirement plan.

The plan is "unlawful, impermissible" under the Civil Rights Act of 1964 because it treats women employees "as statistics rather than individuals," he contended.

At least at this point, the implications of the case of pension plans, most of which treat both sexes alike, generally seem somewhat murky. In a friend-of-the-court brief, however, two groups of actuaries foresaw "significant problems" if sex were to be ruled out as a factor determining the amounts of benefits ot amounts of employer contributions in certain cases.

During yesterday's arguments, Chief Justice Warren E. Burger and Justice William H. Rehnquist wondered aloud if a decision that men and women must pay equally for pensions would entitle men to sue on the grounds that they were forced to subsidize higher total payouts to women. The attorney for Los Angeles agreed that men then would have such a case.

Under the Los Angeles plan, a woman took home about 15 percent less pay than a man with the same salary. Before a new law took effect three years ago, one woman, for example, had contributed $18,671 to the plan, while man of comparable age and job status had contributed $13,275.

Yet, attorney Dohrmann protested, "the woman's rent is the same, her bills at the supermarket are the same."

For the city of Los Angeles, Oliphant contended that the deapartment actually was more generous to women than to men, because it matched each employee's pension payment witha 110 percent contribution of its own - and the women's payment were larger.

He argued that there was no "no discrimination" in the plan. Suppose, he said, that instead of having a compulsory retirement plan, the department simply had paid men and women equally. If the men then went to private insurance companies to buy individual annuities they would pay less than women would - but no one could have accused the department of a civil rights violation, he said.

Oliphant also argued that the plan is valid under a 1976 Supreme Court decision upholding a health plan that provided equal benefits to men and women but excluded pregnancy as a disability.

The government rejected his interpretation of the decision. In a friend-of-the-court brief for Justice Department and the Equal Employment Opportunity Commission, Solicitor General Wade H. McCree said that nothing in the 1976 decision suggests that the Los Angeles plan's "imposition of sex-based burdens on employment is permissible" under the civil rights law.

The decision "considered the disability benefits plan is differentiating between pregnant women and all other nonpregnant persons, including nonpregnant women, i.e., a distinction of a basis other than sex," McCree said. "By contrast, the pension plan . . . classfied and distinguished between two groups of employees explicitly and exclusively on the basis of sex."