Boys were more likely than girls to feel pressure and more likely to think that waiting is a myth, the survey found. It also found a close link between use of drugs and alcohol and sexual activity.
The survey, released Monday by the Kaiser Family Foundation, paints a portrait of youth attitudes about sex and the risk of pregnancy and sexually transmitted diseases.
Teen pregnancy and birth rates have been falling for a decade — a trend that other surveys have attributed to a drop in sexual activity and an increased use of condoms and other forms of birth control.
In an interview with CBSNews.com, Judith Steinhart, Senior Health Educator at Alice!, Columbia University's Health Education Program, said it's much more common than it used to be for teens to be sexually active with intercourse.
"One reason is years ago, when people had fewer contraceptive options, they were forced to have more creative sex lives. Intercourse was a last resort," she told CBSNews.com.
The Kaiser survey, which does not include trend information over time, spotlights areas of concern: Four in 10 sexually active teenagers have taken a pregnancy test or had a partner who did so. A significant minority of young people — about one in six — say that having sex without a condom occasionally is not a big deal. And one in five say they have had unprotected sex after drinking or using drugs.
Other surveys have found that nearly two in three teens will have had sex by the time they graduate from high school. The Kaiser survey shows that many have intimate relationships before that, with more than half of 15-to-17-year-olds saying they have been with someone in a sexual way. Among teens who have not yet had sex, nearly a third say they have been "intimate" with a partner.
"A lot of teens are having sex without pleasure," says Steinhart. "Having it is more important than feeling the pleasure that comes with it. Being able to tell their friends is sometimes more important than any pleasure that they would share with their intimate or sexual partner."
Steinhart adds that some teens are in truly caring relationships. "There are teens who express their tender feelings with their intimate partner in a loving way. Not everybody is having sex to tell their friends or to prove that they're mature."
However, about one in three teens said they had been in a relationship where they felt things were moving too fast sexually.
"Changing social norms and cultural expectations as well as delayed marriage means many young people have multiple sexual relationships in their lifetimes and need the information and tools to make healthy decisions and communicate with their partners," the report said.
Separately, the National Campaign to Prevent Teen Pregnancy was releasing its own study Tuesday examining sex among younger teens. It found that about one in five teens report having sex before they turn 15 years old.
That report, a compilation of data from earlier surveys, also found that younger teen girls who are sexually experienced were more likely than older teens to say they wish they had waited to have sex.
"Parents, program leaders, school officials, community leaders and others need to recognize that sex and dating are important issues for middle school age youth that cannot be ignored," the campaign said.
The Kaiser survey found that boys face particular pressure to have sex, often from male friends — in contrast to the typical portrait of boys pressuring girls.
"There are a lot of expectations for boys to be sexually active," said Julia Davis, senior program officer at the Kaiser Family Foundation, an independent group that studies health issues.
One in three boys ages 15-17 say they feel pressure to have sex, compared with 23 percent of girls. The pressure to drink alcohol was greater for both boys and girls; pressure to use drugs was about even with pressure to have sex.
Overall, 63 percent of all 15-17-year-olds agreed either strongly or somewhat that "waiting to have sex is a nice idea but nobody really does it," with boys 6 percentage points more likely to say so.
The survey also found:
More than eight in 10 teens say that a lot or some people their age drink or use drugs before having sex. Seven in 10 said their peers don't use condoms when they are drinking or using drugs.
About a quarter said that alcohol or drugs had influenced their decision to do something sexual at least once.
More than half of teens believe oral sex is not as big a deal as sexual intercourse, with boys more likely to believe this. Four in 10 consider oral sex "safer sex," although some diseases can be transmitted this way.