I'm trial-running my own response to that.
Same-sex behavior probably has always existed in every society. Agreed? And that raises the excellent question of why gay marriage has never existed in ANY society.
The reason is that there is nothing in a gay pairing which requires the socially and legally enforced "twosiness" which is a part of the central definition of marriage: "for better or for worse, for richer or for poorer, in sickness and in health, til death do us part."
Gay relationships are historically and inherently more fluid than the husband-wife dyad. As not-for-us marchers in Gay Paree put it recently: "We're gayer without marriage."
What is it about the specifically male-female pairing that makes it require the blessings (or, mind you, the burdens) of social, religious, and legal definition and enforcement? It's the fact that this is the only form of sexual union which can spontaneously result in procreation.
No strictly adult-adult relationship requires vows of fidelity except man-woman pair-bonding. This is because adults can negotiate the boundaries of their own relationships: by custom, by consent, or by contract.
Only man-woman couples need the structure of law--- not to bond them to each other, but to bond them to the children which may result from their sexual union. Marriage is the only institution we have which is intended to permanently link a man, a woman, and the children they beget and bear together from their union.
It exists to assert, not the adults' desires, but the children's rights: their right to the provision, care, and sustenance of their natural father and mother.
Why would people with an inherently more fluid gay relationship dynamic, actually require the limitations of a religiously-rooted, historically-fraught, heteronormative, gender-binary legal bond?
Believe me, you're gayer without it.
I agree, of course. Human beings are male/female, two sides of the same coin.
Your elegantly reasoned response would make perfect sense to a sensible person. Unfortunately, we are dealing with the Father of Lies here. They don’t rally want marriage; they want:
• society’s stamp of approval for homosexual behaviors
• money benefits usually reserved for married couples
• the females want to have children without having to remain attractive to a man
• the males want to erode any barriers to living with children and having access to lots of children. If they have to pose as a nice married couple, what’s it to them? Most of the males, at least, do not intend to uphold any of the other norms, such as fidelity.