Sexual health is critical to overall well-being, yet it is challenging and uncomfortable to discuss. Individuals frequently encounter uncertainty about their sexual health as they experience bodily changes, navigate romantic or sexual relationships, and explore their identities. In this study, we called on uncertainty management theory to guide an investigation of sexual health uncertainty. Specifically, we asked how people use social support to manage their sexual health uncertainty in online forums. Grounded theory analyses revealed that negative emotions, avoidance, and pursuing medical care prompted people to seek emotional and informational support online, and support served three functions: integrating information, assessing risk, and strategizing communication. The results point to stigma as a driving force in sexual health discussions online and imply practical recommendations for sexual education, communication about sexual topics, and patient-provider relationships.