This study assessed the risky sexual behavior of a group of adult men and women with sexual experience within the past 1 year who, while in a relationship with a stable partner, reported having sex with another person. The data were collected in 2003 using a nationally representative, multi-stage stratified probability sample (n = 1,271) of women and men in South Korea. Via hierarchical regression models, we assessed the gendered effects of socioeconomic characteristics, health behavior, sex behavior, and safe sex attitude factors. According to the results, for groups with concurrent sexual partnership experience, if the subject was a single person with smoking and drinking who had engaged in sex resulting in unwanted pregnancy, and anal sex initiated before the age of 17, the likelihood of engaging in concurrent sexual partnerships was statistically significant. Furthermore, our gender-elaborated analysis demonstrated that the majority of sexual risk was borne by women. When the partners of concurrent sexual partnerships are categorized by type, the key characteristic of a casual relationship or relationship with female sex workers is one-night-stand sex, accompanied by drinking. This study found that, for groups with concurrent sexual partnerships experience, there is a statistical association between health risky variables and risky sexual behavior. We should attempt to intervene in these concurrent sexual partnerships groups for the effective management of sexually transmitted infections.