Abstract: In Italy, in the latest reform in family law done by the Law No. 76/2016 the duty of marital fidelity was eliminated from the binding obligation for same-sex civil partnerships. Does this consist in discrimination against heterosexual spouses? The aim of this article is to analyze how the concept of the duty of marital fidelity has changed over time. In the past, it guaranteed the legal fatherhood of a wife’s children, whereas now it represents loyalty and respect towards the partner with whom a person has chosen to share his or her life. A comparative methodology helps to understand how this transition occurred and how it has been influenced by religion, science and social evolution.