While the emergence of sex education in the early 20th century is well studied, religious reactions to it remained under-studied. The paper addresses this gap by analysing the emergence of Roman Catholic sex education in the German-speaking area from the turn of the century to the 1930s. It explores official doctrinal guidelines, inner-Catholic debates, and the growing body of Catholic sex education publications. The paper argues that for the Roman Catholic Church, sex education was not a mere pedagogical problem but a topic that raised overarching questions such as how to deal with modern natural sciences, female emancipation, or the secular school system. The findings suggest that lay people and clergy working in pastoral care were the driving force for the modernisation of the discourse vis-à-vis a more conservative church authority.