For sexual health education implementation, program fidelity, defined as whether prescribed curricular components are delivered as prescribed, is critical. Whereas fidelity data is often collected quantitatively with little to no information about what changes were made and why, this mixed methods case study evaluation aimed to understand what kinds of adaptations are being made and why teachers are making them. This was done in order to better support teachers implementing sexual health education in Chicago Public Schools. Twenty-nine class-periods were observed in four high schools to assess curriculum fidelity. Twelve teachers were interviewed about their implementation practices. Teachers made numerous adaptations, primarily aimed to respond to students' needs and interests. Teachers report needing more guidance on how to appropriately make adaptations that maintain fidelity to core content while allowing them to adjust to their own students' needs and interests. Implications for sexual health education program implementation are discussed.