[The following is from Giambattista Vico’s NEW SCIENCE, Book 1, Section 3.]
As for the principle of marriage, some believe that, when free men and women engage in sexual relations without solemn matrimony, as sometimes happens in fact, they commit no wrong under the law of nature. But all the nations of the world reprove this opinion as false by their civilized customs. On the basis of these customs, they all celebrate marriage religiously, and thus define illicit relations as a bestial sin, if in a minor degree. The reason is simple. Parents in such unions are joined by no legal bond, and soon forsake their natural children. Since the parents may separate at any moment, the children find themselves abandoned by both of them, and are exposed to be devoured by dogs. Then, if they are not adopted and raised by the public or private charity of a humane society, such children will grow up with no one to teach them religion, language, or any other civilized custom. Left to themselves, such orphans will turn the world of nations, enriched and embellished by the many fine arts of humanity, into the great ancient forest where Orpheus’ foul beasts, wandering in brutish and abominable error, practised the bestial intercourse of sons with mothers and of fathers with daughters. This incest is the execrable abomination of the lawless world.