People who were sexually unfaithful without their partner's knowledge were less likely to practice safe sex than those who had other sexual relationships with their partner's consent. They were also more likely to be under the influence of drugs and alcohol at the time of the encounter.

In a study published in The Journal of Sexual Medicine, researchers from the University of Michigan, USA, found that condom use for vaginal and anal sex was 27% and 35% lower in sexually unfaithful relationships and drug and alcohol use was 64% higher.

Of the 1,647 people who replied to an online advertisement, 801 had had sex with someone other than their primary partner. Of those, 493 stated this had happened as part of a negotiated non-monogamous relationship and 308 said that they were sexually unfaithful while in a committed monogamous relationship.

"Our research suggests that people who are unfaithful to their monogamous romantic partners pose a greater risk for STIs than those who actively negotiate non-monogamy in their relationship," says lead author Dr. Terri D. Conley from the Department of Psychology at the University of Michigan. "Monogamy can be an effective method for preventing the spread of STIs, but only if couples test negative for STIs at the start of the relationship and remain faithful while they are together. If people do not find monogamy appealing or feasible, they clearly need to think about the risk this poses to their partner and consider whether an open relationship would suit their needs better, and better protect their relationship partners."

"More work is needed in both prevention of and education about sexually transmitted diseases," explains Irwin Goldstein, editor-in-chief of The Journal of Sexual Medicine. "This research is of particular interest because it reveals that monogamous relationships are not always monogamous which can have resultant sexual health implications."