Tucked away in last month's massive Social Security tax bill were two little-noticed provisions significantly enlarging women's rights to Social Security benefits.

They deal with delicate subjects on which Congress seldom legslates directly: divorce and remarriage.

And they denote a major convulsion in the entire Social Security system, brought about by the changing role of women in America and the changing structure of the family.

One change will, starting next year, repeal the "living in sin" rule. For a widow over 60 receiving a pension based on her late husband's earnings, this rule cuts her benefits as much as 50 per cent if she remarries.The result: many aged people live together unmarried. Repeal will benefit about 130,000 widows. (It will apply to men, too, but most retire on their own earnings records and aren't penalized for remarriage.)

The secoocnd change will allow a diiivorsed wife to receive a pension on reaching retirement age, based on the earnings record of her former husband, as long as the marriage lasted at least 100 years and she doesn't remarry. About 45,000 will benefit. The previous requirement was 20 years, which left many thousands of women high and dry when their marriages broke up just short of the 20-year span. Their only hope was to remarry or to build up some meager Social Security credits of their own by working on the remaining 15 years or so before they reached 62.

Women's groups increasingly are demanding, as one House member put it, that Social Security be "desexed," that it be freed of the attitudes toward women that prevailed when the system was set up in the 1930s.

Despite all the efforts in various areas of national life to give women more independence, the advocates argue, Social Security still treats women largely as economic dependents of men instead of enhancing their freedom.

At a time when 45 per cent of the nation's women are working and divorce is shooting up. Social Security still assumes women don't work and do stay married, they say..

"The Social Security system as it stands today deliberately treats men differently then women, and is possibly the worst example of institutionalized sexism in our society," Eleanor Cutri Smeal, president of the National Organization for Women, told a House Ways and Means subcommittee last summer.

As some of the sex-based distinctions or restrictions, and the "punishments" for divorce and remarriage, are cleared away, the women's groups are shooting for much bigger changes. The goal: far more independence from husbands, and far better Social Security protection.

Under existing law, taxes paid into the system generate entitlement to benefits, and the taxpayers's spouse and children derive benefits from that. The women's groups want some form of wage credit - even if a woman never works a day outside the home - that would entitle her to benefits in her own right, not just the lesser benefits derived from her husband's work record.

This concept was endorsed by the National Women's Conference in Houston last November is under study in the Department of Health, Education and Welfare, and is expected to be explored and is expected to be explored by the House subcommittee.

One major proposal, advanced by former Rep. Bella Abzug (D-N.Y.) and Rep. Barbara Jordan (D-Tex.) in different forms, would give a woman who stays home to take care of her children or other relatives a wage credit under Social Security just as if she had worked. The credit might be based on the average wage for women who did work, or on the "economic value of a housewife" - a concept that in 1972 valued the labor of a housewife aged 25-34, for example, at about $6,400 a year.

The cost could be paid by the Treasury (Abzug proposal) or by a tax on the family (Jordon).

This homemaker credit would give every woman her own earnings record, on which she could build when she worked outside the home - for example, after her children were grown.

"With her own earnings record, she would be eligible during her early years for some things nonworking women don't get now as derivative benefits - disability insurance with attendant Medicare benefits regardless of age, survivor benefits for any minor children if she dies.

At 62 she could collect a pension in her own right, even if her husband hadn't yet retired. She could divorce her husband when she chose, knowing that her retirement benefits didn't depend on him.

This system would eventually end the basing of wives' and widows' benefits on husbands' earnings; the women would have their own benefit base.

A second major proposal, sponsored by Reps. Don Fraser (D-Minn.) and Martha Keys (D-Kan.), was first pushed to public attention by the Women's Equity Action League, then headed by Frazer's wife Arvonne. It has the support of the National Organization for Women, American Association of University Women, National Council of Catholic Women and other groups, according to a Fraser aide.

It would simply split a couple's total wages between husband and wife for Social Security crediting purposes. If a husband made $12,000 a year and his wife nothing, each would be credited with $6,000. If he made $8,000, and she made $4,000, again they'd each get $6,000 credits.

This plan would have the same advantages to the wife as the homemakers credit: independence, entitlement to disability benefits and children's benefits, creation of a continuous work record.

Both these plans have some severe problems in common.

The wife's broadened eligibility - particularly for disability benefits - and her continuous earnings record would probably mean much higher costs for overall Social Security system. Workers are already groaning about the much higher taxes voted last month.

The homemaker credit, according to former Social Security Commissioner Robert Ball, presents some problems "to which there are no good answers." For example: how big should the credit b? Should a working wife who also [TEXT OMITTED FROM SOURCE] Who pays? What about a working husband takes care of the house get it too? band who does half the housework in a two-worker family? Should the credit be compulsory or voluntary?

A problem with the Fraser plan is that benefits for a couple could end up lower than now. Under present law, a man who makes $12,000 retiring in 1979 gets $5,090 in annual benefits, plus another "free" 50 per cent if he has an aged dependent wife, for a total of $7,640.

However, if he split the $12,000 50-50 with his wife, the two benefits based on $6,000 would total only $6,350. Another version of the Fraser plan has a higher benefit but is correspondingly more expensive.

Ball discussing the plan in his forthcoming book "Social Security Today and Tomorrow," foresees considerable worker resistance to automatically crediting a wife with half a wage. (In case of divorce, for example, the husband who actually earned the money would end up much worse off than now. Instead of $5,090 he would only get $3,170.)

A third proposal, originally advanced by ex-Rep. Martha W. Griffiths (D-Mich.), deals with a different problem: the fact that a working woman may pay into Social Security for years yet earn a benefit lower than the extra "free" 50 per cent that any woman can obtain simply by being married to a man who retires on his own earnings record.

These working women have repeatedly complained that if they can get a higher benefit free just by being married, then all their contributions over the years are not buying them anything. They might as well not pay in.

The Griffiths proposal, and several similar ones, would simply allow a two-earner family to unite their earnings and then get the same 150 per cent couples' benefit they would obtain if the entire earnings were the husband's. This would mean higher benefits for many couples.

Ball, discussing this problem, said that a woman with her own entitlement isn't buying "nothing" just because her retirement benefit may be low. Rather, she is buying all the things that the Abzug-Jordan and Fraser proposals would provide her.

The current dispute over homemaker credits, wage-credit splitting and "cheated working wives" has its roots in the origins of the Social Security system.

In 1939 and 1940, when dependents and survivor benefits were added to Social Security, only 25 per cent of all women above the mid-teens and 14 per cent of all married women were working. Therefore, said Bell in an interview, the only possible way to protect women against loss of income in old age was to anchor their benefits to husband's wage records.

In the intervening years there has been a revolution in women's work habits. Today, about 45 per cent of all women are working. The divorce rate has doubled. Women's life expectancy, which was 65 years in 1940 compared to about 61 for men, has jumped to about 76 compared to 68 for men.

However, women's labor tends to be more intermittent. They take time out for childbearing. They earn less (the median was $10,031 in 1976 for all male workers, only $4,296 for all female; for full-time year-round workers the figures were $13,455 for men and $8,099 for women.

Because of their intermittent work, at lower-paying jobs, most women collect less than men when retiring on their own Social Security earnings records. Many women's groups say this is unfair.

Yet some Social Security experts believe that, with all its apparent inequities, the system is doing a good job and shouldn't be tampered with too much.

They point out, for example, that about 65 per cent of all elderly persons are receiving benefits of some type. Even though many are widows or wives capitalizing on husbands' earnings, the point is that most are covered and are receiving income protection. That is the real objective of the system.

Almost every plan for improving women's benefits would cost much more, or reduce someone else's benefits, when the system is under attack as already too costly.

Also, according to Ball's forthcoming book women pay 28 per cent of the costs and get 54 per cent of the benefits.

Says one expert who doesn't think the inequities are as great as many women's groups contend: "The real impetus for the Fraser and homemaker bills is to let the women say, 'I have my own pension plan. To hell with you, I'm leaving.'"