Researchers found that among 279 teenage girls they interviewed, many said they'd given in to unwanted sex at some point because they were afraid their boyfriend would get angry.
The findings, published in the Archives of Pediatrics & Adolescent Medicine, indicate that may teenagers - both female and male - need help in negotiating their relationships.
"We need to give guidance to teens on how to communicate with each other," said lead study author Dr Margaret J. Blythe, a pediatrician at the Indiana University Medical Center in Indianapolis.
That means helping girls to take more control over their sexual activity, and boys to understand what constitutes pressure, according to Blythe.
The importance of educating boys, she said, "is often the untalked-about part."
The study included girls between the ages of 14 and 17 who were seen at urban health clinics in Indianapolis. Over about two years, the girls were periodically interviewed about their current relationships, including any instances of unwanted sex over the past three months. Specific questions included: "Would he break up with you unless you had sex?" and "Would he get mad if you didn't want to have sex?"
In all, 41 percent said they'd had unwanted sex at some point. The most common reason was fear that their boyfriend would become angry. Ten percent, though, said their partner forced them have sex when they didn't want to. About 5 percent said they'd had sex after being offered money or gifts.
Girls who reported unwanted sex also reported less condom use, a poorer relationship quality and a higher rate of pregnancy than their peers, the study found.
Other research has shown that unwanted sex, particularly in cases of rape, can lead to depression, anxiety and post-traumatic stress disorder. The current findings, Blythe and her colleagues say, point to sexually transmitted diseases and unintended pregnancy as additional serious consequences.
Most of the girls in the study were black and lower-income, and it's not clear how representative the findings are of the general population. But the results are similar to those of some past studies, according to Blythe and her colleagues.
For example, girls who reported unwanted sex were more likely than their peers to have a partner who smoked marijuana, and other studies have linked drug and alcohol use to forced or unwanted sex.
Substance use can blur the line between consensual and non-consensual sex, the researchers write, and boys who use drugs or alcohol may become "less sensitive" to what their partners want or don't want.