Because advocating same-sex marriage — hereafter, SSM — is undeniably a change to the immediate, local standard*, one common tactic asks: If marriage has changed before, why not again?
To this end, they may undermine monogamy by historical precedent, specifically by invoking polygamy and polyandry; varying social imprimaturs on divorce may be mentioned; Marriage, A History goes so far as to claim spirit marriages, typically between a dead man and a living woman, are also marriages. See how much these things have changed!
And yet there is the haunting echo that polygamy is between a man and several women; polyandry would be between one woman and several men; spirit marriage, so-called, is nonetheless between a man and a woman. Even divorce has never enabled you to marry whomever you please, civilly or otherwise. What tyrant, what barbarian, even claimed to have married someone of the same sex? And what was the reaction? Athens, frequently invoked as sexually enlightened, may have given attraction between men special status and honor higher than marriage, but this only proves that the philosophers made a distinction.
Marriage, even when not understood as a union of one living man and one living woman so long as they both shall live, always united two halves of an altsexual, warring species. Marriage always united Man and Woman.
As if trying to drown out that echo, as terrible as the sound of war drums, SSM advocates will invoke the idea that marriage has been used for a great many things. But if it has been used to unite a kingdom, to gain a dowry, to gain more inlaws or to really tick off your spinster aunt, it has always been between Man and Woman.
Again it comes, and so again SSM advocates bear an idea as a fetish. This time, they insist that marriage has been expressed a great many things. But if it has expressed committment or hormones, rebellion or the status quo, if it comes about only because fathers with shotguns, it has always been between Man and Woman. If there is any form to marriage at all, it is more in the complementarity of the sexes than in fidelity and monogamy, or wedding bells or white dresses, or in children and growing old together. It is more in the complementarity of the sexes even than the modern quasi-Romantic ideal of passionate emotive love.
As come off the coverings, as uncracks the shell, there is the solid atom of what a marriage is. Here is the indissoluble substance, if there is any, to what marriage has been: Man and Woman. Knowing what this represents better minds than mine have said, and is a topic for another time. Knowing what marriage is, and always has been, requires no hop to theology.***
Before you comment, observe that this argument has a limited and specific purpose, and is itself not an argument against SSM. I do not take the tack that historical things are always good, or that rebellion against them always bad. I do not here invoke the proposition about fertility, the natural law or the telos of sex. It is not even a case for marriage. It is merely a rebuttal of one absurdly common tactic shoring up the case for SSM.
To wit, I mean only to point out that advocating SSM is at least a rebellion against a historical reality. It is disingenuous to claim otherwise or even to ignore this part of the argument. If an argument is to honestly propose SSM, it must not only show that the cause is just but that it is just despite also being a rebellion.
* It is a fair human practice in any organized society that we appeal to precedent. If we did not, and did merely as we wished, society would cease to have any cohesion, and lose its organization in a hurry. (This rule of thumb must not be understood as an absolute prohibition on social change, ref. postscript above.)
** Old feature, new readers: Consider reading the About page, and all the way to the end.