The past generation has witnessed a sharp decrease in married couples' share of the nation's households and an explosive increase in single people living alone and unrelated persons living together, the Census Bureau reported yesterday.

But while these trends are continuing, there is a small countertrend in one area, at least in the past year. The increase in the number of unmarried couples stopped, it said.

In a study giving a clear statistical picture of the decline of the traditional family, the bureau said that last March there were 86,789,000 households in the United States, and only 58 percent were married-couple households. In 1970, the proportion was almost 71 percent.

Another 14.3 percent of the 1985 households were also classified as family households because the inhabitants, although not headed by a married couple, were related -- for example, an umarried mother with children. In 1970, only 10.6 percent of all households were in this category.

The remaining 27.7 percent of 1985 households consisted of 20.5 million single-person households and 3.5 million households made up of two or more unrelated persons. In 1970, these two groups accounted for only 18.8 percent of all households.

The bureau said households consisting of a single person or of unrelated persons have had a remarkable rate of growth -- accounting for 48.5 percent of the increase in the total number of households since 1980.

The growth of these households, the bureau said, is "the aftermath of widowhood, divorce and other forms of family dissolution, but many of them are formed for other reasons such as young adults choosing to live alone for a period after leaving their parents' home."

With women living longer because of health improvements and generally outliving their husbands, a large share of the one-person households (11.4 million) consists of elderly widows. Because there are so many widows, the median age for persons living alone was 65.5 for women and 41.4 for men.

At the same time, people have been tending to postpone marriage and child-bearing, so the median age of marriage now is 25.5 for men and 23.3 for women, the highest yet recorded for women, the Census Bureau said.

Though young adults during the 1970s were moving out and setting up single-person households, since 1980 many young adults have been doubling up in the old family nest, possibly to save money because of economic difficulties.

With the growing share of single-person households, the average number of persons per household fell to 2.69 from 3.14 in 1970.

One category that did not increase from 1984 to 1985 was the number of couples living together but not married, which includes some persons without a close personal relationship, such as an elderly widow renting a room to a college student. That figure decreased about 50,000 after more than a decade of growth but remained close to 2 million.

By far the largest proportion of unmarried couples, 1.2 million out of the 2 million, involved householders under 34 years old. Census officials said the lack of increase in the unmarried couple group probably reflects the low birth rates of the past generation and the resulting decline in the number of young people.