Relationships and Sex Education (RSE) has been subject to controversy historically and several studies have analysed the challenges and barriers to delivering it in schools. Recently, however, a new global phenomenon has sharpening debate with the emergence of anti-gender movements and their discourse on 'gender ideology'. The focus now is less on the issue of delivering RSE in school, than the possibility of there being such a thing as RSE. This controversy derives from the responses of reactionary social, religious and political groups in many European countries, but organised globally. These groups appeal to parents' right to veto RSE activities in schools. This article examines this phenomenon starting from the thesis that the ultimate aim of current anti-gender mobilisations is not so much to undermine sex and gender equality policies, but democracy itself. To do so, it first examines the actions of anti-gender movements and the 'gender ideology' they claim exists. It then moves to analyse school-based RSE, discussing debate about delivering RSE in schools and how anti-gender movements have hardened this debate, with special attention paid to 'parent'-led controversies in Spain and Europe.