Argument #1: Marriage has always been between a man and a woman.
This isn’t technically an argument. There’s an argument embedded within it, though, which can be stated as follows:
(1) If an action doesn’t have historical precedent, then it’s wrong;
(2) Gay marriage doesn’t have historical precedent; therefore,
(3) Gay marriage is wrong.
But this argument commits a logical fallacy, as it’s plainly an appeal to tradition. Appeals to tradition fail for the simple reason that an action’s being traditionally accepted in no way guarantees its moral rightness. For instance, until the early twentieth century, it was an American tradition that only men could vote. Until late in the nineteenth century, it was a tradition that only white men (and before that, only white men who owned land) could vote. Countless similar examples could be given.
Argument #2: If homosexuals are allowed to marry, then what’s to stop people from marrying minors, from marrying animals, from marrying relatives, from marrying more than one person, etc., etc.?
This argument commits the slippery slope fallacy. It’s in no way clear that legalizing gay marriage would lead to legalized bestiality, etc. Although (many) conservatives seem to believe that marrying someone of the same gender is the moral equivalent to marrying animals or marrying little children, most Americans don’t see things this way. Most Americans believe that gay marriage should be legal for the same reasons they believe heterosexual marriage should be legal—because both unions are agreed upon by consenting adults. The same, of course, cannot be said about marrying animals or marrying children. Thus, there’s no reason to believe that legalizing same-sex marriage would lead to human-pet marriages and the like.
Argument #3: Homosexuals already HAVE the same rights as heterosexuals: just like heterosexuals, they have the right to marry someone of the opposite sex. (Therefore, granting homosexuals the right to marry would be granting them “special rights.”)
This argument fails because it presupposes a completely inadequate definition of equal rights, a definition that I don’t think I’ve ever before heard, certainly a definition that is almost never used in American political discourse. When we say that people have equal rights, we do not mean that everyone is free to do as the majority does. We mean that, within reason, everyone is free to make their own decisions and live according to the dictates of their own consciences, even if the decisions they make are different than the decisions of the majority.
So, for example, if Jews had the right to worship Jesus—and only Jesus—we wouldn’t say that Jews and Christians had equal rights. Or if Democrats had the right to vote for Republicans—and only Republicans—we wouldn’t say that Democrats and Republicans had equal rights. In the same way, it’s absurd to say that gays and straights have equal rights since they both have the right to marry members of the opposite sex, and only members of the opposite sex.
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 I heard these arguments from a Facebook friend who clearly doesn’t know Facebook etiquette, or etiquette in general I’m afraid.[1a] You know the type, the all-too-sure-of-himself, all-too-opinionated religious dogmatist who thinks it’s his duty to “straighten out” everyone else’s beliefs. And since I do have Facebook etiquette (at least most of the time), I don’t respond to such blowardedness on Facebook, but these particular comments so annoyed/infuriated me that I felt the need to respond somewhere. So here you go, dear blog readers, all six of you, or however many of you there are left.
[1a] Of course, now that I think about it, I guess it’s possible that this person has almost exclusively like-minded friends and so they don’t think that posting anti-gay comments on Facebook will offend anyone. Because I have Facebook etiquette and thus don’t make it a habit of sharing my opinions there, I guess it’s possible that they’re not even aware that I don’t agree with them. Anyway.
 As for marrying your relatives goes, well, I don’t know about you, but I don’t know any brothers and sisters looking to tie the knot. As for polygamy, I couldn’t care less if a man wants to marry multiple women. Seems like a good deal for the women: you get extra help with the housework,[2a] you’re not being pressured to sleep with the disgusting slob every night,[2b] etc.
[2a] Not that I’m saying women should be the ones who do the housework. I just made this comment because women still tend to do more housework than men.
[2b] Not that all men are disgusting slobs who repulse their wives. I just tend to think that a man who wants multiple wives might—just might—and I could be wrong here—fall under this category.
 By “within reason,” I mean actions that do not harm others. And despite what religious fundamentalists claim, same-sex marriage does not harm others, at least no more than opposite-sex marriage does.