Researchers at the University of Michigan found that people who jump into bed with someone without their partner's knowledge are unlikely to practice safe sex, with one explanation that they are more likely to have had a drink.
However, those in open relationships are less likely to get an STD -- possibly because they are sober when they sleep with someone else and are more sexually aware, they found.
"Our research suggests that people who are unfaithful to their monogamous romantic partners pose a greater risk for STDs than those who actively negotiate non-monogamy in their relationship, lead researcher Dr Terri Conley was quoted as saying by the Daily Mail.
Scientists already know that monogamy can be an effective method for preventing STDs spreading, but only if the couple test negative for infections at the start of a relationship.
In the study, the researchers posted an online advert to see if monogamy can be a preventative method for STDs, and if people in an open sexual relationship practice safe sex.
Out of the 1,647 people who responded, 801 admitted to having sex with someone other than their partner -- with 493 of these claiming this had happened as part of a negotiated non-monogamous relationship.
A further 308 stated they were sexually unfaithful to their partner while in a committed monogamous relationship. Results showed condom use was 27 per cent and 35 per cent lower respectively for cheats, compared to those in an open relationship.
The researchers also found that drug and alcohol use was 64 per cent higher for cheating partners in a monogamous relationship.
Monogamy can be an effective method for preventing the spread of STDs, but the results showed that it is effective only if couples test negative for such infections at the start of the relationship and remain faithful, Dr Conley said. (More)