Based on the theory of normative social behavior, this research investigated the moderating role of three forms of sexual stigma - enacted, felt, and internalized - in the relationship between perceived descriptive norms and intentions of gay and bisexual men to engage in three preventive behaviors during the 2022 mpox outbreak in the U.S. Through an online survey conducted among 439 gay and bisexual men in September 2022, we observed that participants' perceived descriptive norms regarding mpox preventive behaviors among gay and bisexual male friends and among gay and bisexual men in general were both positively related to their intentions to adopt the recommended mpox preventive behaviors. However, these norm-intention associations were moderated by sexual stigma. Specifically, the positive associations between friend descriptive norms and intentions were significant only when participants held low internalized sexual stigma. Except for the behavior of practicing hand hygiene, the positive associations between general descriptive norms and intentions were significant only when participants held low felt sexual stigma. Enacted sexual stigma did not interact with either perceived descriptive norms among friends or those among gay or bisexual men in general. Theoretical and practical implications of the study are discussed in the end.