More than half of young, unmarried Americans who consider themselves vulnerable to herpes are changing their sexual behavior to avoid catching it, though only about 10 percent of adults list venereal diseases as those they fear most, according to a Washington Post-ABC News public opinion poll.

The finding confirms recent suspicions of many medical experts that the herpes epidemic in this country may have slowed the sexual revolution, at least the casual choice of sexual partners.

"That's a large change. I don't think that would have happened a few years ago. It shows a large concern about contracting the disease," said Dr. Lawrence Corey of the University of Washington, one of the country's leading herpes researchers. "People are thinking twice about the one-night stand."

"It's probably the most serious thing that's happened to sex in a long, long time. People are thinking twice," agreed a 28-year-old driver's education instructor from Redwood City, Calif., who sees equal concern about herpes in the teen-agers he teaches.

The poll, one of the first to sample national public opinion on herpes, was conducted by telephone from Sept. 14-19 on a random selection of 1,505 adults aged 18 and older.

A 23-year-old herpes sufferer from Roswell, N.M., who participated in the poll, said, "It's caused me to have a more conservative attitude toward sexual inclinations and has caused much depression and frustration."

The survey found that almost nine out of 10 adults of all ages listed other diseases as those they fear most. Cancer remains at the top, with heart disease a distant second. Only 10 percent of those polled listed herpes and other venereal diseases.

Genital herpes, the nation's most common sexually transmitted disease, represents a growing public health problem and a distressing physical and psychological burden on those who have it or fear they will get it. Although new drugs to help treat the symptoms are coming on the market, the disease has no cure.

Government estimates suggest that between 5 million and 20 million Americans may be affected, with as many as 500,000 new cases reported each year. Some experts say as many as 20 percent of young adults may have herpes.

The Washington Post-ABC News Poll found that only 1 percent of those contacted acknowledged having genital herpes. Yet 8 of 10 persons had heard of the disease, most within the last two years, and 12 percent said they know someone with genital herpes. Many said they knew at least six herpes victims.

Like their older counterparts, the majority of unmarried people in the group up to age 37 maintained that their "behavior has been such that" they had "no real fear of getting herpes."

Corey said this finding suggests that many people are denying it could happen to them.

The greatest awareness of and concern about the disease was found in younger people who saw themselves as most vulnerable to the disease, presumably because of greater sexual activity.

Twenty-two percent of all young unmarrieds said they had changed their behavior to "avoid the risk of contracting herpes." Nearly 14 percent said they were "not concerned enough" to change their behavior.

This means that within this sub-group of individuals who considered themselves at greatest risk, nearly 60 percent were actively trying to avoid getting herpes, while one-third were not.

Follow-up interviews with some of those who have changed their behavior suggested that they are doing so by avoiding casual sex.

"I'm really careful with who I go to bed with in the first place. I ask questions. Who has he been to bed with before me? What type of persons are they? Has he ever had any problems? I'm not afraid to ask," said a high school senior in Moncks Corner, S.C.

This 18-year-old is worried about the "personal effect," should she get herpes. "I'm sure it's a very aggravating thing. I'd be worried someone would not want to go to bed with me if I had it," she said.

A 20-year-old Philadelphia furniture delivery man said he watches "who I go out with. I don't just hop in bed with anybody. I have to go out a while and see if it feels right. . . . Hey, I'm not going to get that. It's a bad disease."

Those in the more "vulnerable" younger group are twice as likely to know someone with herpes and more likely to consider the disease worse than other venereal diseases, the poll found.

Nearly half of the adults surveyed thought genital herpes is more serious than syphilis, while 23 percent thought it less so and 11 percent considered them about the same. Nineteen percent had no opinion.

Medical experts view untreated syphilis as far more serious than herpes, but most cases of syphilis and gonorrhea, both caused by bacteria, can be treated with antibiotics.

Genital herpes, caused by a virus related to the one that causes the common cold sore, seems to be viewed more seriously than other venereal diseases because it is incurable.

After the first painful episode in which sores or blisters break out in the genital region, the virus hides in the body until it causes another outbreak. Subsequent bouts tend to be shorter and less serious, but the victim is most likely to pass the disease to others during this infectious stage.

Herpes also has more serious long-term consequences, having been linked to development of female cervical cancer. Pregnant women may infect their infants during childbirth. The disease is highly fatal to babies but may be prevented by cesarean section deliveries.

Contrary to widespread impression, the poll also found that middle- and working-class adults seldom differ in knowing someone with herpes or in their concern about avoiding it.

Many herpes stories have portrayed the disease as more likely to strike well-educated people in the middle and upper classes. A 1979 poll by the California-based American Social Health Association of members with herpes supported this view, but it has been regarded an unrepresentative sample.

"It's always been in the lower socio-economic groups, but there has been more increase at the upper socio-economic end and they are the most vocal," Corey said.

"The most change is in the college students and the middle class where there is greater openness about sex," said Dr. Andre Nahmias, an Emory University researcher who has found herpes to be relatively constant in the segment of Atlanta's poor population he has followed since the early 1960s.

The poll also found that:

* Three-fourths of those surveyed thought there should be a required herpes test before marriage, while 17 percent disagreed. There is no simple, cheap test for diagnosing the disease, although one may be ready for marketing within a few years. Many experts question the desirability of such a test since there is little that can be done to treat victims.

* The majority of adults are concerned personally about the overall increase in venereal diseases in this country, although they are changing their view that sexually transmitted diseases are a problem of the promiscuous. While nearly 50 percent felt that those who contract such diseases do so because of contact with many sexual partners, that "wouldn't make much difference" in the opinion of nearly 40 percent of those polled.

* When asked whether they would "tend to avoid associating" with someone they knew had herpes, nearly two-thirds of those polled said they would not, while nearly one-third they "would avoid" a herpes victim. The question did not specify whether this meant sexual or all personal contact.

* While only a small number of people admitted they had herpes, those who did were most likely to be young women living in cities or suburbs. Most thought they had not transmitted the disease to others. One-fourth said the disease had limited or stopped their sexual activity.