Understanding situations that increase HIV risk among men who have sex with men (MSM) requires consideration of the context in which risky behaviors occur. Relationships are one such context. This study examines the presence and predictors of unprotected anal intercourse (UAI) in the past 3 months among 566 MSM couples. A majority of couples allowed sex with outside partners. Overall, 65% of the sample engaged in UAI with primary partner, including nearly half of discordant couples. Positive relationship factors, such as attachment and intimacy, were associated with an increased likelihood of UAI with primary partner. Meanwhile, 22% of the sample engaged in at least one episode of UAI with an outside partner, half of whom were discordant or unknown HIV status outside partners. Higher levels of HIV-specific social support, equality, and sexual agreement investment were significantly associated with a decreased likelihood of engaging in UAI with a discordant or unknown HIV status outside partner. HIV-positive men in discordant relationships had two and one half times the odds of having UAI with a discordant or unknown HIV status outside partner as their HIV-negative partners. Many MSM in relationships, including some in serodiscordant ones, engage in UAI with primary partners. Potential explanations include relationship closeness, relationship length, and agreement type. In addition, relationship context appears to have a differential impact upon UAI with primary and outside partners, implying that prevention messages may need to be tailored for different types of couples. Prevention efforts involving MSM couples must take into account relationship characteristics as couples balance safer sex and HIV risk with intimacy and pleasure.