Citizenship education programs promote political participation by young people. These programs risk misrepresenting politics to students by encouraging them to believe that there are universally accepted principles which govern the definition of citizenship and who is entitled to participate in its various dimensions. The article argues that issues surrounding access to participation in citizenship are inescapably political, such that if young people are to be encouraged to participate in politics they should be helped to recognize the political significance of debates about inclusion in participation. If inclusion in citizenship itself is a focus of political contestation, then participation in politics can involve disputes about the scope of inclusion. Some students may face issues regarding their own access to participation. Students ought to learn that, since the outcome of political contests cannot be guaranteed, their own defeat is a possibility. The article draws on "agonistic" political theorists such as Chantal Mouffe to show that contests over participation cannot be resolved through appeal to principles such as equality and justice, because the meaning and application of these principles are politically disputed. The debate over same-sex marriage is presented as an example of a legitimate political contest over access to citizenship, in which the life chances of participants are at issue, and defeat for some is inevitable.