The classic argument of ignorance. "I don't understand why we should have civil marriage, therefore we shouldn't have civil marriage.
Every lawyer used to know William Blackstone's Commentaries inside and out. It was a requirement of being admitted to the bar. Law schools have failed the nation.
The duty of parents to provide for the maintenance of their children is a principle of natural law; an obligation, says Puffendorf, laid on them not only by nature herself, but by their own proper act, in bringing them into the world: for they would be in the highest manner injurious to their issue if they only gave the children life, that they might afterwards see them perish. By begetting them therefore they have entered into a voluntary obligation, to endeavor, as far as in them lies, that the life which the;y have bestowed shall be supported and preserved. And thus the children will have a perfect right of receiving maintenance from their parents. And the president Montesquieu has very just observation upon this head; that the establishment of marriage in all civilized states is built on this natural obligation of the father to provide for his children; for that ascertains and makes known the person who is bound to fulfill this obligation.
Commentaries, Book 1, Chapter 16 emphasis in the original.
Which is why divorce should not be allowed legally.