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In 2003, Belgium opened marriage to same-sex couples before granting them access to adoption in 2006 on the grounds, among others, of the antidiscrimination principle. Nevertheless, legislators have maintained differentiated filiation rules according to the spouses’ sex. Indeed, the presumption of paternity only applies to heterosexual married couples. The aim of this article, based on Belgian parliamentary discussions, is to understand why this principle, which establishes a man’s paternity towards his wife’s child, was not modified at the same time, as it was done in other countries involved in similar legislative debates. More generally, the analysis of the presumption of paternity and its various sociological and legal stakes provides the opportunity to question the current functions and meanings of the filiation.