The long and helpless infancy of man requires the combination of parents for the subsistence of their young
 David Hume, An Enquiry Concerning the Principles of Morals (1751). copy citation

Context

“as the league has here a peculiar UTILITY, the conditions of union have a peculiar sacredness and authority, and a violation of them would be regarded as no less, or even as more criminal, than any private injury or injustice. The long and helpless infancy of man requires the combination of parents for the subsistence of their young; and that combination requires the virtue of chastity or fidelity to the marriage bed. Without such a UTILITY, it will readily be owned, that such a virtue would never have been thought of. [Footnote:” source