Many teenagers and young adults fail to use condoms consistently, regardless of whether they have sex with a serious or a "casual" partner, a new study suggests.

Researchers found that among more than 1,300 15- to 21-year-olds, those with casual sex partners had unprotected sex just as often as those in serious relationships — about 20 times over the previous three months, on average.

Those who had casual sex were more likely to use condoms at least some of the time, the study found. But because they had sex more often, they ended up having unprotected intercourse just as frequently as their peers in steady relationships.

The findings point up two different problems, according to the researchers.

"Unfortunately, this reveals that teens may overestimate the safety of using condoms most of the time with a casual partner and underestimate the risk of unprotected sex with a serious partner," lead study author Dr. Celia Lescano said in a statement.

Lescano and her colleagues at Brown Medical School in Providence, Rhode Island, report their findings in the Journal of Adolescent Health.

The study included 1,316 15- to 21-year-olds from three large U.S. cities. Overall, 65 percent said they’d had sex only with a serious partner over the past three months; the rest said they’d had at least one casual partner.

Those with casual partners said they used condoms 47 percent of the time, versus 37 percent among those in steady relationships. Still, young people in both groups had unprotected sex at about the same frequency.

For study participants in relationships, one of the problems seemed to be their perception — correct or not — that their partner did not want to use condoms.

It’s important, Lescano’s team writes, that teenagers be taught that consistent condom use is necessary, regardless of who their partner is or how long they’ve been in the relationship.

They say young people who worry their partner will be turned off by condoms need to be reminded that most people accept condom use — and that consistent use lowers the risk of sexually transmitted diseases for both partners.