The National Association of Insurance Commissioners expressed unanimous support yesterday for a model automobile insurance classification system that eliminates sex and marital status as standards for determining premium payments.

But the 50 commissioners who regulate the insurance companies in their states stopped short of recommending that legislatures ban sex and marriage as criteria.

After a week-long meeting in Chicago, the state officials decided to withhold final judgment until they have studied the test model's economic impact as well as actual experience in the three states - Hawaii, Massachusetts and North Carolina - that haave ceased using the sex and marriage classifications. The model system would add such variables as driving record, miles driven and number of years licensed. A final recommendation is expected next December.

Asked to characterize yesterday's action, NAIC president Hosea P. Hudson of Indiana called it "a mature step forward (instead of) universally abolishing a system before we know if a substitute system works."

Last year the NAIC staff had called for eliminating the sex and marriage criteria, but the commissioners backed away as the result of vehement industry opposition. At that time Hudson urged the industry to come up with alternative classifications.

Rather than do so, the industry calculated the cost of elimination alone without substitution, and came up with some startling figures. It estimated that premium rates for young single male drivers would decrease 14 percent while those for young single female drivers would go up 26 percent and those for young married couples, 37 percent. One insurer quoted a $422 a year increase in the premiums for a Chicago couple.

Noting that young single males have 59 percent more losses than married men also under 25, an industry spokesman said, "To ignore these statistics by eliminating sex and marital status as rating variables would force low-risk young drivers to pay more than the facts warrant in order to subsidize the losses of the high-risk young, single male motorist."

In related news, the Alliance of American Insurers this week also announced the results of its latest survey on the cost of automobile repairs. It estimated that if a standard 1979 car priced at $5,741 were totally wrecked and every part replaced this year, the rebuilt car would cost $26,418. The ratio has remained steady over the past three years.

At that rate damage to less than one fourth of a car's parts would cost more to repair than the car is worth. And even moderate damage to a three or four year old auto may not make it worth repairing.

The calculations, done by Auto Damage Appraisers of Chicago, were based on the dealer invoice cost of a Chevrolet four-door Impala, loaded with options. Dealer preparation charges, markup and sales taxes were not included. On the repair side, typical dealer discounts of 10 to 15 percent for parts were not included, nor were sales taxes. All parts were new and labor was figured at $14 an hour.

So the actual ratio might be smaller than the 4.6 to 1 for replacement parts. But, of course, this is a hypothetical example anyway, the purpose of which is to emphasize safe driving. CAPTION: Graph, The High Cost of Fixing Crash Damage, The Washington Post