Background: Detailed information on the sexual behavior of bisexual, non–gay-identified men and the relationship between same-sex behavior and HIV/sexually transmitted infection (STI) incidence is limited. This study provides information on the sexual behavior with male partners of non–gay-identified men in urban, coastal Peru and the relationship of this behavior with HIV/STI incidence. Methods: We analyzed data from 2146 non–gay-identified men with a baseline and then 2 years of annual follow-up, including detailed information on sexual behavior with up to 5 sex partners, to determine the characteristics associated with bisexual behavior. Discrete time proportional hazards models were used to determine the effect of self-reported sex with men on subsequent HIV/STI incidence. Results: Over the 3 study visits, sex with a man was reported by 18.9% of men, 90% of whom also reported sex with a female partner. At baseline, reported bisexual behavior was associated with other sexual risk behaviors such as exchanging sex for money and increased risk of HIV, herpes simplex virus type 2, and gonorrhea. The number of study visits in which recent sex with men was reported was positively correlated with risk of other sexual risk behaviors and incident HIV, herpes simplex virus type 2, and gonorrhea. Recent sex with a man was associated with increased HIV/STI incidence (hazard ratio, 1.79; confidence interval, 1.19–2.70), after adjusting for sociodemographics and other sexual risk behaviors. Conclusions: Given the prevalence of recent sex with men and the relationship of this behavior with HIV/STI incidence, interventions with non–gay-identified men who have sex with men and their partners are warranted.