The criminalization of cohabitation (living together outside of marriage) in
Article 412 of Law Number 1 of 2023 concerning the Criminal Code has sparked
heated debate. The background of this study is to analyze the conflict between the
demand for the affirmation of public morality and the guarantee of the Principles of
Legal Certainty and Lex Certa in the context of a constitutional state, particularly in
relation to the formulation of offenses that touch on the private sphere. The main issue
examined is whether the formulation of the offense in Article 412 of the Criminal Code
has fulfilled the principles of Legal Certainty and Lex Certa, as well as the impact of
its non-fulfillment. The research method used is normative legal research with a statute
approach and a conceptual approach. The data was analyzed qualitatively to test the
consistency of the formulation of Article 412 with the principles of criminal law,
especially Lex Certa. The results of the study concluded that Article 412 of the Criminal
Code does not fully comply with the principles of legal certainty and Lex Certa because
the key phrase “living together as husband and wife” is not defined operationally. This
ambiguity has implications for the emergence of normative uncertainty and the risk of
excessive discretion in law enforcement, which threatens privacy rights and triggers
over-criminalization, thus requiring legislative reform