This is where you go wrong. That is precisely the gay argument.
If marriage is a social construct, it can change. The only thing that makes marriage not susceptible to change is the fact that it's not socially constructed at all. It's not man-made. It's given, with the rest of human nature, and childbearing is part of that nature, unless someone frustrates the course of nature.
The shared Judeo-Christian tradition to which you appeal has no concept of marriage as a thing that can be severed from childbearing. Modern day Jews or Christians asserting to the contrary are not a continuation of that tradition, but a departure from it.
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.