THURSDAY, Jan. 20 (HealthDay News) -- Many young American couples can't agree on whether they've decided to have sex only with each other, a new study shows.

Oregon State University researchers analyzed data collected from 434 heterosexual married and non-married couples, aged 18 to 25. In 40 percent of those couples, one partner said the couple had agreed to be monogamous while the other partner said there was no such deal.

Even among couples who agreed that they had decided to have sex only with each other, nearly 30 percent had broken that agreement, with at least one partner having sex outside the relationship, the findings indicated.

The study also found that couples with children were less likely to have a monogamy agreement in place and married couples were no more likely than other couples to have an explicit monogamy agreement.

In fact, the authors noted, the only relationship aspect related to sustained monogamy was commitment. A scale from one to five was used to assess whether participants viewed their relationship as permanent, and the higher they scored on the commitment scale, the more likely the couple was to have a sustained monogamy agreement.

The findings will be published in an upcoming online edition of the Journal of Sex Research.

"Other studies have looked at perceptions related to monogamy, but this is really the first one that explores the discussions that heterosexual couples are, or aren't, having about monogamy," research associate Jocelyn Warren said in a university news release.

"Couples have a hard time talking about these sorts of issues, and I would imagine for young people it's even more difficult," study colleague Marie Harvey, a professor of public health, said in the news release.

"Monogamy comes up quite a bit as a way to protect against sexually transmitted diseases. But you can see that agreement on whether one is monogamous or not is fraught with issues," she added.

More information

The U.S. Centers for Disease Control and Prevention outlines how to protect yourself from sexually transmitted diseases.