I remember, when taking a course on logic in college, that we learned the names and problems with several logical fallacies. We learned to avoid over-generalization and false causality. We learned how to detect a straw man argument, ad hominem name-calling, and circular reasoning. There was one fallacy that, though I could understand that at times it might be misused, I had qualms about. That was the slippery slope argument.
I would like to relabel this argument and, in turn, argue for the revocation of its fallacy status. I would like to divide the argument into two parts: a fallacy of the "Inevitable Conclusion" and a valid argument of "Contributing to a Trend."
It is certainly illogical to assume that because A leads to B, and B can sometimes lead to C, and C usually leads to D and etcetera, doing A leads to Z. That is something I would put into the Inevitable Conclusion category -- taking one step east doesn't lead me from San Francisco to Boston. The idea on its face is laughable and obviously illogical to anyone with a scintilla of reason.
However, it is equally irrational to deny that sociological change occurs in short, halting steps that lead to, over time, vast upheavals in the fabric of society. One step won't get me from San Francisco to Boston, but if I'm heading toward a cliff it would be quite foolish to argue that each step is inconsequential. The slow and steady march in our society away from stable and responsible two-parent homes has been going on for a long time, and with each step we have heard that we are illogical to assume that the next step we are asked to take will lead any further. We went from no-fault divorce to sexual libertinism, which led to us asking Uncle Sam to support the offspring of the behaviors we allowed. This led to a marked destabilization of the biological family. We were asked to tolerate, then liberate, and now practically to venerate homosexuality. This led to media saturation of the behavior, which has caused a misperception among the general public, which now believes that homosexuality is so common that gays have become a statistically significant minority. (Most presidential polls have a margin of error larger than the percentage of homosexuals among the population.)
This has led to the call to label marriage a legal bond between any two people, of either the same or different genders. Yet, as our opposition calls us irrational for opposing the idea and casts us as primitives, they fail utterly in making a clear and logical case, in unemotional terms, for the benefits to society as a whole that such a redefinition of marriage would bring. Words like "fairness," "equality," "dignity," and etc. are amorphous labels that can be applied to anything if stretched far enough. They are not the components of a logical argument. "Why not?" is actually a known logical fallacy -- an argumentum ad ignorantium, or "appeal to ignorance." The lack of evidence against something does not count as evidence for it.
Opponents of "garriage" are able to make an argument from definition. Marriage is and always has been a male/female phenomenon. When media figures try to use polygamy as a wedge against us, we can simply thank them for making our point for us; polygamy is, after all, simply a phenomenon in which one male has simultaneous marriages with multiple females. Thus, even in cultures with different marriage practices, those practices have always been between a male and female pair. Thanks Mr. Liberal; you've made my argument for me.
Let us not fall for the same trick again and again. The only logically valid argument being made is ours. We argue that since the legal definition of marriage is a relationship between a man and a woman, same-sex marriage is simply a contradiction in terms. If gays want to invent for themselves some other formal relationship structure, they may -- this is a free country. Few of us care if they can own property together, have hospital visitation rights and/or inheritance rights. (Actually, all of these things can be done via a legal contract without any additional laws being passed.) What we object to is being forced to include apples in the citrus category because those apples would really like to be equal to oranges.