As to my rational argument against homosexuals destroying marriage I would start with the unalterable fact that homosexuals have the exact same rights as heterosexuals they just don't wish utilize these right preferring their chosen fetish instead. Willing abrogation of rights is not a legally rational reason to ask for more rights. Then it can be further argued that there is no real reason for the cultural institution of marriage to be overthrown when everything they are asking for can be handled with civil unions.
Lastly and most importantly it should be recognized that marriage as an institution was formulated by our shared Judeo-Christian western ethos. As it is the basic building block of our civilization it should enjoy respect and protection from it being undermined. It predates our Government by centuries and as there is no constitutionally enumerated power for government to regulate marriage as it has no legitimate say over it, period. Their only involvement should be in enforcing the marriage contract.
Marriage is a vital social construct and child bearing is an important facet but its essential role can be filled without the marriage being a productive union.
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.