Nearly 70 % of HIV+ men who have sex with men (MSM) are estimated to have contracted HIV from a main partner. We examine whether condom use varies by relationship configuration, including open relationships with and without cheating. 656 MSM in committed relationships were recruited through a sexually explicit social networking website. Of the 55 % of MSM who had anal sex with a non-main partner in the past 90 days, two-thirds did not use a condom. Adjusting for covariates, MSM in relationships characterized as open with cheating versus monogamous were more likely to have unprotected anal sex with both main and non-main partners. MSM who perceived that their partner played around or cheated were more likely to have unprotected anal sex with a non-main partner. Prevention messages should attempt to reduce cheating and increase personal responsibility for protecting partners from HIV. Messages should be tailored to reflect open and monogamous relationships.