Jocelyn Warren and Marie Harvey of Oregon State University say a study that involved 434 young heterosexual couples ages 18-25 found in 40 percent of couples -- both married and unmarried -- only one partner says the couple agreed to be sexually exclusive, while the other partner said there was no agreement.
Even among the couples who agreed they had an explicit agreement to be monogamous, almost 30 percent say at least one partner had sex outside the relationship, the researchers said.
Interestingly, couples with children were less likely to have a monogamy agreement in place, while married couples were no more likely to have an explicit monogamy agreement than unmarried couples, the study said.
"Relationship variables appear to be related to monogamy," Harvey said in a statement. "But factors such as marriage and children did not increase the likelihood that the couple had agreed to monogamy."
This lack of communication between heterosexual couples is leading to unintended risks of sexually transmitted diseases, Warren said.
The findings are to be published in the Journal of Sex Research.