New research has shown 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.
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 for vaginal and anal sex was 27 per cent and 35 per cent lower respectively for cheats, compared to those in an open relationship.The researchers also found drug and alcohol use was 64 per cent higher for cheating partners in a monogamous relationship.
The University of Michigan researcher Dr Terri Conley said: '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.
'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.'
Irwin Goldstein, editor-in-chief of The Journal of Sexual Medicine where the study is published, said: 'More work is needed in both prevention of and education about sexually transmitted diseases. 'This research is of particular interest because it reveals that monogamous relationships are not always monogamous which can have resultant sexual health implications.'