Harassment is a persistent problem in contemporary online environments, with women disproportionately experiencing its most severe forms. While critical scholars posit that online gender harassment may be linked to men's anxieties about fulfilling normative masculine gender roles, this relationship has not been examined by empirical research. We survey 264 young men between the ages of 18-24 about their masculinity anxieties and their perceptions of harassment directed at a woman on Twitter. We find that men who perceive themselves as less masculine than average men report higher endorsement of harassment. Further, we find that the relationship between masculinity anxieties and harassment endorsement is mediated by men's adherence to masculine norms and toxic disinhibition. We interpret these results through the lens of social media's specific affordances, and we discuss their implications for technology designers and other practitioners who wish to better detect, prevent, and remediate online harassment by accounting for the role of gender.