The marital presumption is deeply rooted in Anglo-American law: a husband and wife are assumed to be the father and mother of any child born during their marriage. With the advent of sophisticated genetic testing, no-fault divorce and changing family structures, however, American states are now questioning the continued validity of the presumption. Paternity can be determined with certainty and much of the stigma associated with the circumstances of a child’s birth has disappeared. In the face of these changes, the presumption has been exposed as a legal fiction without a simple meaning, even as it continues to confer parenthood: in all states, married couples, gay or straight, who walk out of the hospital with a child are legal parents. This article explores how states struggle to reconcile the strong public policies supporting marriage with new knowledge about biological facts.