Finlayson’s emergent rhetorical political analysis is used in this article to examine arguments made in support of the criminalisation of both consensual and non-consensual adult familial sexual activity, as part of a key governmental review and report on sexual offences (Setting the Boundaries (the Report)). The argument presented in this article is that a ‘synthetic necessary truth’ was created in the justification of the criminalisation of an activity that would otherwise be permissible within the framework proposed for the review. This article examines the creation and formation of arguments in support of the criminalisation of incest made during the pre-legislative political process. It does this by employing a rhetorical political analysis, which does not dismiss the rhetorical properties of political speech but rather demands these become central to the review. By drawing attention to the Reports’ rhetorical properties, the investigation moved away from ideas about the correctness of criminalisation of incest and instead placed the focus firmly on the structure and quality of the arguments used.