Organized sport has largely been, and remains, segregated by sex. A long history of cultural norms and gender socialization discouraged women from participating and competing in sport. The conflation of sex and gender is produced by and further reinforces notions of biological essentialism that denies the full inclusion of trans people. Anchoring our analysis in epistemologies of ignorance, this paper critically examines the online discourses surrounding Lia Thomas’ participation in NCAA women’s swimming events to understand how the swimming community constructs notions of sex, gender, and fairness. We reviewed 1696 online comments on articles about Thomas’ participation in women’s events on a globally renowned swimming website, SwimSwam. We argue that biological essentialism functions as an epistemology of ignorance, where the dominant group actively reject or misrecognize scientific, historical, and sociological knowledges to justify the exclusion of trans women swimmers. In turn, biological essentialism erases equity issues within sport and, thus, effectively works to reproduce the privilege position of particular groups—in this case cisgender women and men—by protecting their opportunities to swimming success.