People in open and other consensually nonmonogamous partnerships have been historically underserved by researchers and providers. Many studies group such partnerships together with nonconsensual nonmonogamy (NCNM) under the banner of “concurrent sexual partnerships.” Discrimination from service providers poses a substantial barrier to care. Responding to such concerns, this investigation explored sociodemographic correlates with open relationships and associations between relationship structure and sexual risk, HIV/STI testing, and relationship satisfaction in a nationally representative probability sample. Data were drawn from the 2012 National Survey of Sexual Health and Behavior (n = 2270). We used multinomial logistic regression to identify correlates with relationship structure, and linear and logistic regression to investigate associations between relationship structure and testing, condom use, and relationship satisfaction. Eighty-nine percent of participants reported monogamy, 4% reported open relationships, and 8% reported NCNM. Males, gay/lesbian individuals, bisexual individuals, and those who identified as “Other, Non-Hispanic” were more likely to report open relationships. Bisexual individuals and Black, Non-Hispanic participants were more likely to report NCNM; older participants were less likely to do so. Participants in open relationships reported more frequent condom use for anal intercourse and lower relationship satisfaction than monogamous participants. NCNM participants reported more HIV testing and lower satisfaction. Identities, experiences, and behaviors within open and other consensually nonmonogamous populations should be regarded as unique and diverse, rather than conflated with those common to other relationship structures. There is a need for greater awareness of diverse relationship structures among researchers and providers, and incorporation of related content into educational programming.