This study aims at ensuring the existence and sufficiency of regulations on marriage registration based on the Islamic law and the Indonesian legislative regulations. It also seeks to identify the reasons and causes for why some people choose not to register their marriage at the authorized state institution as well as the harms that come with such decision. In addition, the necessary efforts to discourage sirri marriage (religiously valid yet unregistered marriage) and to prompt people to register their union are also laid out. To achieve these purposes, this study employs the juridical-normative method with a sociological approach in the writing. The findings are as follows: (1) registration is important and necessary to regulate marriages in Indonesia; (2) registration does not determine the validity of a marriage because it is not a basic principle or requirement of matrimony; (3) Islam allows a government to require marriage registration if it is beneficial or good for the community, and if it can help them avoid any form of oppression, damage, and risk; (4) Indonesia already has a sufficient set of legislative regulations on marriage registration, although some of them need to be clarified further; (5) different reasons and causes for not registering a marriage at the authorized institution include: religious validity is considered adequate, the official registration process is deemed impractical, pre-marital pregnancy, the marriage is part of a polygamy, the marriage is interreligious, the marriage involves underage individual(s), and the punishment is simply not serious enough; (6) unregistered marriage is prone to risks such as a slander or suspiciousness by other people, unrecognized by the state, bearing the status of a mistress or having an extra-marital child, vulnerability to conflicts, and disordered and disorganized household management.