question: Social media platforms are increasingly considering models to incentivize creators to publish high quality content on their platforms. As a result, social media content creation has transformed into a form of gig work for some creators. In order to better design social media platforms to support this labor, we need to understand professional creators' motivations. In this work, we present a qualitative interview study of the motivations of $22$ U.S. OnlyFans creators. OnlyFans is a subscription-based social media platform that is unique in that it is primarily associated with sexual content (although it is not marketed as such) and thus creators are positioned at the intersection of professional content creation and sex work, exposing them to a unique set of potential challenges. Beyond the typical motivations for pursuing other forms of gig work (e.g., flexibility, autonomy) our findings highlight three key factors explaining the rapid growth of OnlyFans despite the potential stigma of sexual content creation: (1) societal visibility and mainstream acceptance of OnlyFans, created through a combination of celebrity hype and the design of the platform itself; (2) platform design: affordances for boundary setting with clients, privacy from the public, and content archives, which together create a labor environment participants viewed as better than other forms of gig work, a natural avenue for sexual expression, and enabling monetization of existing content, audiences, and skills; and (3) the pandemic, which led to both high demand for immediate income while waiting for -- or after running out of -- unemployment benefits, and increased free time, which increased general demand for pornographic content.