Recent studies among men who have sex with men (MSM) have found that the majority of HIV transmission results from sex with a main partner. One factor likely to affect the risk of transmission is the type of agreements the couple has regarding sexual behaviour within and outside the relationship. This study recruited 732 Internet-using MSM through Facebook banner ads. Participants completed an online questionnaire regarding demographic characteristics of the respondent and their main partner, the sexual behaviour of the couple, the existence of a sexual agreement, and the strength of investment in that agreement. The Pearson chi-square test was used to assess the association between sexual agreements (categorized as open, closed, or none) and the predictive variables. Respondents' investment in their sexual agreement was measured using the sexual agreement investment scale (a composite score ranging from 0 to 52). Ninety-one percent of respondents had some form of sexual agreement in place with their main partner. The presence and type of sexual agreement was found to be strongly associated with many characteristics of the individual and couple, including the respondent's HIV status, length of time with the main partner, having unprotected anal intercourse with a man other than their main partner, and happiness in the relationship. Increases in the strength of respondents' investment in their sexual agreement were found to be associated with newness of the relationship, relationship happiness, having a closed relationship, and decreases in risky sexual behaviour. This study offers further evidence of the important role that sexual agreements play in male couples. The overwhelming prevalence of sexual agreements and their association with relationship happiness and risky sexual behaviours has important implications for future HIV prevention and control strategies, including the implementation of couples voluntary counseling and testing.