It seems like in every discussion about gay rights, opponents raise the specter of sex with youngsters, siblings, sheep, goats, dogs, corpses, and just about every other forbidden--and sometimes not-so-forbidden--union. As Justice Scalia wrote in his dissenting opinion to Lawrence v. Texas, "State laws against bigamy, same-sex marriage, adult incest, prostitution, masturbation, adultery, fornication, bestiality, and obscenity are likewise...called into question by today's decision." Imagine that: if we're soft on homosexuality, next thing you know they'll be allowing masturbation.
Commentators usually call this type of reasoning a slippery slope, but it is more of a reductio ad absurdum. The difference is subtle but important. With a slippery slope, you're opposing a policy on the grounds that it will lead to worse policies--regardless of logical connection. With reductio ad absurdum, you're attempting to expose the flaw in an argument by showing that its logic leads in unacceptable directions. The classic example is the cliché response to peer pressure: "If your friends jumped off a cliff, would you follow them?"
People often use reductio ad absurdum to hide the weaknesses of their own arguments. It's pretty easy, with the help of a little self-delusion, to shut your mind to nuanced distinctions. Then all you have to do is tell your opponent, "If you hold X, you might as well hold Y." While he's busy explaining why he doesn't hold Y, he doesn't notice that you've shifted the discussion away from X.
Over at DovBear's blog, I recently had a debate with a fellow who calls himself Lawyer-Wearing-Yarmulka. He reiterated an argument he made two years ago in which he suggested that the philosophy of consent underlying gay rights could be used to justify sex between men and cows. His point was that we never demand consent from cows for anything else we do to them. If we can turn them into burgers without their consent, why shouldn't we be allowed to have sex with them?
LWY fails to understand that consent has a particular significance with regard to sex acts, which doesn't apply to other types of acts. By their nature, unwanted sexual experiences are highly traumatic. The notion that there's something inconsistent about opposing cruelty to animals while condoning slaughter of animals is an argument I would expect from a PETA member, not a Bible-believing Jew.
I'm astounded by LWY's suggestion that the law against raping a cow is entirely for the human's benefit, not the cow's. (It brings to mind Lord Macaulay's statement, "The Puritan hated bearbaiting, not because it gave pain to the bear, but because it gave pleasure to the spectators.") This certainly is not the common view in our legal system, where bestiality is typically classed under "animal abuse." While that may differ in some ways from the traditional perspective, that just goes to show that our society has over time shown an increasing concern for the treatment of animals, even if it is still far from perfect.
Notice how far this discussion has drifted from its original topic. In that whole time, LWY didn't offer even one justification for his position on gay rights. All he did was make an egregiously poor analogy, but because it concerned treatment of animals, an area of public policy that is controversial and by no means perfectly consistent, he was able to keep the discussion pointed away from himself.
I've repeatedly encountered this sort of evasive attitude from critics of gay rights. Instead of explaining their own point of view, they spend an inordinate amount of time trying to catch defenders in an inconsistency regarding other sexual deviances. What's especially surprising is that they tend to jump on the least relevant examples, where there is clearly a victim.
Ironically, LWY dismissed the importance of incest, a much better example to create the sort of reductio ad absurdum he was trying to establish. Let's say an adult brother and sister become lovers. Let's say they're not going to have kids: they use birth control, or one of them is infertile, or the woman is past menopause. Their affair would still be illegal in most places. Yet it is a purely victimless act between consenting adults, and so, by the standard argument in support of gay rights, it should be allowed.
What would happen if we did allow it? We might then have to make provisions to ensure that incestuous lovers never have children. Further, we would have to deal with the competing argument that they have a right to have kids, even ones who will likely have genetic defects. And I haven't even mentioned the marriage issue. This is all a big can of worms to open up in the name of consistency (according to Wikipedia, this form of incest is extremely rare), but it is something our society might have to face, if we are to remain committed to the principle that people have the right to pursue their own happiness as long as it doesn't hurt others.
Some gay rights activists, of course, would object to this example on the grounds that incest is "sick" and "disgusting." That's what critics of gay rights want the defenders to concede, that there are certain acts that just fall outside the bounds of civilized behavior, even if they seem to harm no one. But it's unlikely that this example will persuade anyone to abandon a belief in gay rights. This reductio ad absurdum at best reveals an inconsistency in the views of some advocates, but it doesn't come close to dismantling the philosophy behind gay rights.