With reference to the unsuccessful attempt of the Labour Government to make sex education a statutory part of the National Curriculum, this paper argues in favour of making liberal sex education compulsory at all state schools. First, the main characteristics of a liberal sex education are briefly explained. Promoting the virtue of respect for every adult's right of sexual self-determination is presented as one of its central aims. Then the paper shows that state enforcement of liberal sex education is justifiable to reasonable citizens in several ways and therefore meets the liberal criterion of political legitimacy. Finally, the relevant clauses of the Bill of the Labour Government are briefly evaluated. (Contains 7 notes.)