Although international proclamations increasingly recognize legal sex beyond binary categories, domestic legislation is lagging behind. Germany and the Netherlands are the first countries in Western Europe where courts have ruled in favour of a third option. While the German constitution was amended to guarantee a third option, the Dutch government has refrained from implementing comparable legislation. What explains this difference? This article draws on parliamentary debates, newspapers and statements from activists to follow the trajectory of the third option. Building on literature on intersex rights legislation and the diffusion of LGBT rights norms, we find that an established progressive reputation can lead states like the Netherlands to take a back seat, while mobilization by activists was crucial in establishing the self-determination of legal sex on the German political agenda. Our study contributes to unravelling the complex ways in which new norms shape – or fail to shape – domestic political outcomes.