One stock “argument” against same-sex marriage is that legalizing it will put us on the slippery slope to bestiality. That is, if the Lone Ranger can marry Tonto, then he can marry Silver. This line of “reasoning” is easy enough to defeat.
First, this is an example of the classic slippery slope fallacy. Second, there is fact that if allowing different-sex marriage between humans does not lead to or warrant bestiality, then it would follow by analogy that allowing same-sex marriage between humans would not lead to or warrant bestiality. After all, if Adam marrying Eve does not warrant Adam marring a snake, then Adam marry Steve would not do so either.
While the bestiality argument is typically presented as a fallacious slippery slope, it is worth considering whether or not a proper argument can be presented that would show that allowing same-sex marriage entails that bestiality must also be accepted. Obviously, merely claiming that allowing same-sex marriage will lead to human-goat marriage is not enough. What would be needed would be logical reasons that we cannot accept same-sex marriage without being force by consistency to allow human-animal marriage.
Perhaps the most plausible way to argue for this is to begin by contending that same-sex marriage is justified by the principle that a person can marry anyone he wants to marry. This would, of course, justify same sex marriage: if a person can marry anyone he wants to marry, then he can marry another man. And a woman can marry another woman. It would also seem to justify human-animal marriage: if a person can marry anyone he wants, then he can marry a goat. As such, if we justify same-sex marriage on this principle, then it would also justify human-animal marriages. It would also justify human-rock marriages, human-iPad marriages and so on. A person could, on this principle, marry anything.
Now, if it is assumed that a person can marry anyone he wants, then this would also include marrying people who do not want to get married, people who are already married, and even Catholic nuns and priests.
Obviously enough, this principle leads to absurd results. As such, if this were the justifying principle for same-sex marriage, then there would be an excellent reason to reject same-sex marriage. However, if there is another principle (or principles) that would justify same-sex marriage while avoiding absurdity, then this principle could be sensibly used.
One obvious avenue of inquiry is to consider the principle that justifies different-sex marriage. While some might assume that different-sex marriage needs no justification, that would seem to beg the question. Naturally, if what justifies different-sex marriage would also apply to same-sex marriage, then there would not be a principled way to forbid one while accepting the other. However, if the justifying principle for different-sex marriage did not apply to same-sex marriage, then one could be allowed while the other is consistently forbidden.
One approach that people have taken is to argue that different-sex marriage is justified by a principle involving natural procreation. This principle would, obviously enough, not apply to same-sex marriage. However, this principle would lead to its own absurd results, namely that different-sex couples who could not have children or choose not to have children would not be permitted to marry. As such, unless we are willing to forbid such people from being married, then the procreation justification must be abandoned.
Once the procreation principle is out, there seem to be no non-ad hoc or non-question begging principles left that would allow different-sex marriage while forbidding same-sex marriage. For example, if a principle involving love is used, that could apply to different-sex and same-sex marriage (and, of course, we obviously do not take love to be a necessary condition for legal marriage). As another example, if someone claims that the principle is that men can only marry women, this would beg the question. It would be on par with arguing that mixed-race marriage is forbidden because the principle is that a person can only marry a person of the same ethnicity.
One worry at this point is that if any principle that warrants different-sex marriage would also warrant same-sex marriage, then it would seem that we would slide into human-animal marriage. Fortunately, this can be avoided in a principled manner.
Intuitively, marriage is a legal and moral agreement that requires the consent of both parties. Animals cannot, obviously enough, even understand marriage let alone provide consent. As such, a human cannot marry an animal. An animal can no more marry than it can make a promise or tell a lie. As such, same-sex marriage can be allowed without accepting a slide to human-animal marriage.
It might be countered that by taking marriage to require consent I am engaged in an ad hoc or question begging defense. After all, one might say, if marriage can include a man marrying a man, why can it not include a lack of consent and comprehension on the part of one partner, such as a goat? After all, if marriage is being redefined, why not redefine it completely?
The obvious reply is to note that if marriage can include a man marrying a woman, why can it not include a lack of consent and comprehension on the part of one partner, such as a goat? That is, if marriage is allowed, why not allow it for everyone and everything? However, if marriage (like debating or lying) requires certain capabilities (such as the ability to understand the relationship and consent to it), then humans can marry humans but not animals.