As legislation to bar sexual discrimination in determining insurance premiums and benefits comes down to the wire, the insurance industry is stepping up its opposition. The mark-up session by the House Energy and Commerce committee, originally scheduled for this week, has, as a result, been delayed until next week.

"We're back to square one," said sponsor James J. Florio (D.-N.J.), who thought until yesterday he had an agreement between the insurance industry and civil rights and women's groups. "It puts into question the good faith of the industry spokesmen," he added.

Florio's remark referred to yesterday's action by the American Council of Life Insurance, an industry trade group, in reversing its previous stand. ACLI, which represents most of the major insurers, decided on March 10 not to oppose future use of "unisex" actuarial tables that would not distinguish between men and women policyholders. In return it asked Congress to eliminate provisions in the bill that would have made unisex rates retroactive. Retroactivity is included in the bill, though. At the time, ACLI stood virtually alone in the insurance world in its acceptance of unisex rates.

However, many of ACLI's members were so outraged at the board's position, adopted against the advice of a task force, that a petition was signed to call a special meeting. At that meeting yesterday, the vote by company representatives asking ACLI to change its stance was 389 to 102. The organization then altered its policy and decided to support continuation of the present system of sex differentiation for individual insurance contracts.

However, it agreed to support equal contributions by employes and equal benefits in group insurance contracts. Employers could continue to use sex-based tables to determine their own costs in providing benefits.

ACLI President Richard Schweiker said yesterday the organization was reversing only its tactics and strategy, not its principles, which the former U.S. senator added was "not unusual in political life."

At a press conference, several industry leaders made the economic case against retroactive equalization of benefits. They said it would require an increase in 147 companies' reserves of $13.4 billion. Consequently, 21 companies would become technically insolvent and another 50 would have their net worth cut in half if the bill passes in its present form.

Florio said yesterday that the bill's proponents had agreed on some modifications that would address the financial problems and had sent the proposals to the meeting. Instead of considering them, the industry reverted to a hard line, he said. He added he was not sure whether or not he would support the introduction of constructive amendments when the bill comes up next week.

On Tuesday, a coalition representing a dozen life and property-casualty companies inaugurated a letter-writing campaign similar to that successfully used by the banking industry in the fight to repeal withholding of interest and dividends. The coalition has hired the Hannaford Co., a public relations firm to send out model letters to hundreds of thousands of consumers to oppose this "damaging" legislation. The case against unisex rates is couched in economic terms: most people will spend more on insurance if it passes, particularly young female drivers, the industry says. It asks the recipient to write his or her congressional representative urging a no vote on H.R. 100, which "can help only a small special interest group." A spokeswoman explained that that referred to the 2 percent of women annuitants who are expected to gain from the bill.

In addition, several insurance companies, including Geico and Nationwide, have begun sending letters on their own to policyholders urging them to write Congress in opposition.