Just imagine you are on the witness stand being asked these questions? If marriage is a given why do so many people have children outside of marriage? If Marriage is a given how do you explain how different cultures have radically different family constructs if they have any similar constructs at all if? Do you support divorce for infertile marriages? Do you support divorce for those past their child breeding years or, for that matter, should people outside of their fertile years (or otherwise infertile) people be allowed to marry? What is the purpose of a marriage that beyond the respective spouse's fertile years? If marriage is intrinsically about childbirth should childbirth outside of marriage be illegal? If having children is the primary purpose of a marriage, would a marriage that resulted in 5 children be a better marriage than one that resulted in two children? Does it matter who raises the children once they are born if the marriage is primarily about having children? Historically or Biblically has marriage been legitimately or customarily terminated because of infertility (in the Judeo-Christian culture)? Historically and or Biblically has marriage been customarily prohibited between men and women who could no longer have children (in the Judeo-Christian culture)? Historically or Biblically (in the Judeo-Christian culture) has the number of children borne been more important the how well they were raised?
It is against these sort of poorly thought out arguments that homosexual marriage advocates have had so much success.
It is a matter of fact that marriage and the family have been around longer than any human law.
There is something called natural law. Philosophers, including philosophers of law, have heard of it. It is not necessary that every marriage be fertile; it is enough to observe that man and woman are ordered to each other, that the union of man and woman is ordered to childbearing, and that childbearing among human beings is ordered to a stable family in which man and woman remain together for the good of the child. This is how human life was passed on before there was any such thing as law. Without this observance of natural law — of man attracted to woman, of their having sexual union and subsequently remaining together as a domestic unit — the human species could not have survived. Comments about old age and are irrelevant, as the parents still need each other in old age after the children are on their own, and inter-generational bonds ensure that the elderly are not discarded like yesterday’s trash. Comments about infertility are irrelevant. Infertility is a disorder, in the same way that a medical disorder that causes blindness does not somehow cause the eye to be ordered not to sight but to walking or digestion. Man and woman are ordered to each other even if they can’t make babies together: in both body and mind they fit together and complete each other.
Infertility deliberately chosen through the self-mutilation of chemical “castration” or otherwise is an abuse of nature. That is why it’s wrong for Obamacare to cover contraception: because human fertility is a sign of good health, not a disease needing to be cured.
Marriage’s definition argued from natural law, and the facts of our existence as evidence of a supernatural act of creation outside the competence of natural science to explain, are obvious to any reasonable person willing to egage in systematic thought.