The article is devoted to the role of administrative courts in ensuring the legal security of an individual in the area of family law. The aim of the analysis is to demonstrate the guarantee of the preservation by the Polish constitutional legislator of the traditional form of marriage as a union of a woman and a man and the family created on this basis (Article 18 of the Constitution of the Republic of Poland). The article discusses the essence of legal security and the role of administrative courts in ensuring it, as well as the guarantee nature of the regulations of Article 18 of the Constitution of the Republic of Poland. Against this outlined background, the thesis on the inadmissibility of institutionalization of same-sex unions is argued. Problems resulting from the judicial practice of administrative courts related to the transcription of a child's birth certificate are also discussed. The scientific research was conducted by using traditional methods of jurisprudence, primarily the formal-dogmatic method. The linguistic, systemic and teleological interpretation of Article 18 of the Constitution of the Republic of Poland was applied. The directions of operational interpretation resulting from the case law of administrative courts were discussed. The views found in the literature on the subject (doctrinal interpretation) were taken into account, too. According to the author, the perspective of legal security strengthens the arguments stemming from the linguistic and historical interpretation of Article 18 of the Constitution and allows for a better justification of the inadmissibility of institutionalizing same-sex unions by means of a statute. Such a procedure seems particularly useful in the face of the growing tendencies in the law of other countries to eliminate the traditional form of marriage as a union of a woman and a man.