A key to understanding regulation through private intermediaries is how third‐party actors are selected and controlled. This paper examines the question in prostitution policy, a value‐loaded policy field that stimulates regulators to carefully select private intermediaries to avoid regulatory capture. By means of a novel data set on prostitution policy in 25 OECD countries (1960–2010) as well as with a comparative case study on two German states, the paper discovers that the responsibilization of private intermediaries is a slowly diffusing phenomenon, accompanied by strong public oversight. Moreover, the selection of private regulatory intermediaries is an ideology‐driven process in which the congruence in (moral) goals is key for the establishment of any relationship, while regulatory capacities are secondary. Thus, private intermediaries generally rule under a “shadow of moral hierarchy” in prostitution policy. This emphasis on shared moral goals enriches the young research on regulatory intermediaries with a largely disregarded selection criterion, which is able to reduce the risk of regulatory capture by private actors in delicate regulatory areas.