Debate: Does Gay Marriage minimize Straight Marriage in any way?
Side: No, It Does Not

The word "marriage" precedes religion, and so does its concept.If I am correct in understanding that by "precedes" you mean "pre-dates," you're going to have a hard time supporting that point (although I agree with both of your other assertions). Religious behavior is tens of thousands of years old; its seeds can be found in earlier hominids -- ritual burial of the dead, for example. While no religion currently in existence is that ancient in its own right, religion itself is a very old part of human societies. But we don't know a whole lot about the formalization of relationships into "marriages" prior to the introduction of writing, because while we can find actual physical evidence of religious behaviors, we must rely more on records to track the history of marriage as an institution. We know that "marriages" do pre-date writing as an innovation in human culture, but I don't think that we have good evidence as to just when the development of "marriage" as an institutional relationship came about.The word "marriage" is very much younger than either the social institution of marriage or the social institution of religion: "The modern English word 'marriage' derives from Middle English mariage, which first appears in 1250-1300 C.E. This in turn is derived from Old French marier (to marry) and ultimately Latin marītāre (to marry) and marītus (of marriage)." (The Wiki.)However, the origins of the word and the custom aren't really all that relevant since, as you say, "the definition of marriage has already changed multiple times throughout history--both around the world and in America." In terms of the legal definition especially -- which is all we really care about in relation to gay marriages -- this is entirely correct. The legal definition and requirements of marriage have changed many times to accomodate our sense of social justice. You can't marry more than one person at once. You can't marry someone under a certain age limit. You CAN marry someone of a different race. And so forth.What it really comes own to is whether there is a strong social policy in law that justifies denying gay folks the right to marry other gay folks, and since Lawrence v. Texas I think the argument that such a policy exists has been pretty seriously undermined.As evinced by this debate and just about every other debate on the subject, the primary argument against allowing gay marriages is rooted in a modern-day breed of conservative Christianity, and while said conservative Christians are trying harder and harder to argue that their faith is a basis for acts of government, by the terms of our Constitution and our legal tradition, religion is just not a basis for acts of government.Allowing gay marriage, as many other people here have noted, would not change one single thing about straight marriages. It might offend some folks' religious sensibilities, but that's just life in America -- that sort of goes hand-in-hand with the whole "separation of church and state" thing. People's religious sensibilities get offended all the time, and there's most certainly no legal right to not ever have your personal beliefs offended. That whole idea is pretty antithetical to our notions of basic liberty rights like freedom of speech, press, association, etc. And as you say, "civil rights aren't about satisfying opposing parties [but] about social equity and justice"; and nobody is infringing on the rights of conservative Christians to do or believe anything by allowing gay people to marry.