We sought to examine how condom use was differentially reasoned by gay, bisexual and other men who have sex with other men (GBM) in Ontario, Canada. Data were derived from a community-based study of GBM who completed an anonymous online questionnaire in 2014. Participants qualitatively described reasons a condom was used or not at their most recent anal sex event. Qualitative responses were thematically coded non-exclusively and associations with event-level and individual-level factors were determined quantitatively using manual backward stepwise multivariable logistic regression. Among 1,830 participants, 1,460 (79.8%) reported a recent anal sex event, during which 884 (60.6%) used condoms. Reasons for condom use included protection/safety (82.4%), norms (30.5%), and combination prevention (6.2%). Reasons for non-use were intentional (43.1%), trust (27.6%), unintentional (25.7%), and other strategies (19.6%). Event-level substance use was associated with all non-use reasons: e.g., more likely to be unintentional, less likely to be trust. Condom non-use with online-met partners was associated with more intentional and unintentional reasons and less trust reasons. Non-white and bisexual GBM were less likely to explain condom use as a norm. Participant-partner HIV status was an important predictor across most condom use and non-use reasons: e.g., sero-different partnerships were more likely to reason condom use as combination prevention and condom non-use as trust, unknown status partnerships were more likely to reason non-use as unintentional. Condom use among GBM is a multi-faceted practice, especially with increasing antiretroviral-based HIV prevention. Future interventions must adapt to changing GBM (sub-)cultures with targeted, differentiated, culturally-appropriate, and sustained interventions.