A study published in The Journal of Sexual Medicine, entitled "Unfaithful Individuals are Less Likely to Practice Safer Sex Than Openly Nonmonogamous Individuals," has determined just that. Researchers Terri D. Conley, Amy C. Moors, Ali Ziegler, and Constantina Karathanasis undertook the study in order to determine whether sexually unfaithful individuals or negotiated non-monogamous individuals would be more likely utilize safer sex methods. In their introduction, they state:
Given the prevalence and harm of sexually transmitted infections (STIs), there is a need to examine safer sex strategies in the context of romantic relationships and extradyadic sexual encounters . . . little research has addressed the sexual health ramifications of sexually unfaithful partners and members of other high-risk nonmonogamous lifestyles.
Researchers gave an anonymous, online sexual health questionnaire to several hundred sexually unfaithful individuals and individuals with a negotiated non-monogamous agreement. In the end, sexually unfaithful participants demonstrated significantly lower rates of risk reduction behaviors in both their primary relationships and their extradyadic sexual experiences. They were also less likely to undergo frequent STI testing and to discuss safer sex concerns with new partners.
Unfortunately, access to the study is restricted to those with institutional access, a society membership, or those who wish to pay for a 24-hour period of access, but the abstract can be found online.