"Despite having more lifetime sexual partners, people in open relationships were not any more likely to have an STD than those in monogamous relationships. Not only that, the people in open relationships were actually much safer about sex. They wore condoms with all their extra partners and got tested much more frequently. They were also less likely to lie to their primary partner about an extra-sexual dalliance."
http://fusion.net/story/218221/monogamy-versus-open-relationships-std-study/
http://fusion.net/story/218221/monogamy-versus-open-relationships-std-study/