The classification of a state of affairs under a legal category can be considered as a kind of condensed decision that can be made explicit, analyzed, and assessed using argumentation schemes. In this paper, the controversial conflict of opinions concerning the nature of "marriage" in Obergefell v. Hodges is analyzed pointing out the dialectical strategies used for addressing the interpretive doubts. The dispute about the same-sex couples right to marry hides a much deeper disagreement not only about what marriage is, but more importantly about the dialectical rules for defining it.