About 70 percent of unmarried young women living in metropolitan areas have had sexual intercourse by age 19 -- a big increase over four years ago, spurred largely by an increase in sexual activity among white teen-aged girls.

Among unmarried young men in metropolitan areas, about 78 percent are sexually active by age 19.

Those are the key findings of a new study of teen-agers' sexual activity released yesterday by John Kantner and Melvin Zelnick, population experts at the John Hopkins School of Hygiene and Public Health. The study was published in the latest issue of "Family Planning Perspectives" magazine of the Alan Guttmacher Institute.

The study is based on a 1979 survey of more than 2,600 young men and women in metropolitan areas of the United States. It shows more clearly than any other numerical survey the course of the sexual revolution of the 1970s among unmarried teen-aged women, revealing a remarkable upsurge in premarital sex.

A survey taken in 1971 indicated that nearly half of all young unmarried women had sexual intercourse by age 19. By 1976, the figure had risen to 59.5 percent. In the latest survey it had jumped to 69 percent. (All these figures would be even higher if married teen-agers were included.)

The sharpest increases are among white unmarried women.In 1971, only two in five had had sexual intercourse by age 19. By 1976, the figure had risen to just over half, and by 1979 the figure was 64.9 percent.

Sexual activity among unmarried black women, though it has always been greater than sexual activity among unmarried white women, hasn't increased that sharply. The 1971 survey found that 80 percent of black unmarried teen-aged women had had coitus by age 19. The latest survey puts the figure at about 88 percent.

The survey of unmarried teen-aged males, the first ever taken, found that rates of sexual activity weren't much different for whites and blacks.

The survey showed that about a third of the teen-aged women who have sexual intercourse get pregnant. Some then marry, but of those who remain single, only about half have the baby, down from about two-thirds a decade ago. About a third have abortions and the rest miscarry. Kantner, in an interview, said the proportion of teen-aged women who become pregnant and marry before the end of the pregnancy is declining, from 33 percent in the 1971 survey to 15 percent in the most recent one.

The authors said the pregnancy rates among teen-agers are higher than a decade ago, even though there appears to be more use of contraceptives.

One reason for this, the editors speculated, is that teen-agers are using less effective methods of contraception. "There was a considerable increase in use of what is probably the least effective method -- withdrawal," they said.

The survey showed that most teen-aged women engaging in premarital sex reported they used contraception, but about 36 percent said withdrawal was the method, a doubling of the use of that form of contraception since 1976. About a third reported use of the condom. The pill, which had been used by a third of sexually active teen-aged women four years ago, was being used by only 19 percent in 1979.

The report showed that the percentage of premarital sexual activity was lower among girls younger than 19. The authors found that in 1979, half of all teen-aged girls between age 15 and 19 had engaged in premarital intercourse, compared with 30 percent in 1971. Most of this rise resulted from increased rates among white girls.

Kantner speculated that the reason for increased teen sexual activity is "more permissiveness" in social attitudes and the fact that young people now are left "more to their own devices." He said he doesn't know why rates for black girls have always been so much higher than for whites.