Laws against incest were abolished by the Constituent Assembly when it adopted a new penal code in September-October 1791. Incest was simply not mentioned in this code.

In his presentation of the newly drafted penal code to the Constituent Assembly, Le Pelletier de Saint-Fargeau commented that it out-lawed only “true crimes” and not “those phony offenses, created by superstition, feudalism, the tax system, and despotism.” Although he did not list the crimes “created by superstition” - meaning the Christian religion - they undoubtedly included blasphemy, heresy, sacrilege, and witchcraft, and also quite probably bestiality, incest, pederasty, and sodomy, none of which was mentioned in the penal code. By dropping any mention of these former offenses, Revolutionary legislation simply passed over in silence acts that had once, at least in theory, merited the most severe penalties.

Like the 1791 Penal Code, the 1810 Penal Code created under Napoleon did not include "imaginary" crimes, thereby legalising them by omission.

Source: Homosexuality in Modern France, Bryant T. Ragan, Jr. and Jeffrey Merrick, 1996.