The conversation arose in response to a recently publicized instance of polyamory — a trio of women whom the press said had “married.” This led to some “I told you so” fulfillment of the slippery slope: the promise that permission for same-sex marriage would open the floodgates to all sorts of other sexual variations. I’ve addressed this argument, and the logical fallacy of which it is an instance, elsewhere, and won’t repeat my comments here beyond the simple evidence that polyamory made its first appearance in the seventh generation from Adam, so blaming the gays seems an effort to close a barn door opened by someone else.
However, this was not the thesis that engendered the discussion. It was the more nuanced suggestion, “The arguments used in support of same-sex marriage can also be used to support polyamory.” This is a slope of a different slip, a logical fallacy so far as concerns addressing the validity of the arguments in question. That does not mean the phenomenon does not happen — a logical fallacy may still describe a true situation; but it remains a fallacy because it does not address the underlying argument.
As I noted on Facebook,
The thesis takes the form: Argument X in favor of Y is wrong because it can also be used in favor of Z. In the present instance, the claim is that arguments advanced in favor of same-sex marriage fail because those same arguments can be used to support, in this case, polygamy. There are a number of problems with this thesis. Most importantly, it does not in fact disprove the validity of Argument X, but merely observes that the same argument may be employed in another case. Mere antipathy to that other case (which may reflect antipathy towards the first case) is in itself irrelevant. In reality, arguments that support things one holds to be good can also be used to support conclusions one feels are bad.
Let me raise a case where I think the thesis is true, even if fallacious. The libertarian argument in favor of same-sex marriage (or anything else, as it is less an argument than an ideology) takes the form, “People should be allowed to marry if they love each other and are doing no harm to others.” The same argument can be applied to polygamy, and very likely has been. But even though that is the case, it doesn’t actually prove the argument to be wrong, in either case. A whole separate debate on the virtues of libertarianism would need to be entered; and I think most people are neither fierce absolute libertarians nor equivalently doctrinaire authoritarians. In practical terms most people would, I think, given the popularity of another ideology, utilitarianism, home in on whatever alleged “harm to others” might result from any given action. (And however popular and common, a debate on the virtues of utilitarianism would also need take place!) So I concur that a libertarian argument may in fact have wider application than intended — but it may still apply in relation to the action and the harm that are the real subject of debate. If one wishes to debate the principle of liberty or utility themselves, that will have to be a separate discussion.
Getting back to arguments that I have actually encountered in the same-sex marriage debate, one of the principle arguments against it revolves around procreation. Again, I’ve dealt with the merits of that argument elsewhere at considerable length and won’t repeat it here except to note that the overlap between procreation and marriage is incomplete, on both sides.
But the proffered example of procreation can serve as a case in point in the larger question of arguments in favor of things one likes being used to support things one does not. For while procreation is cited as one of the “causes” for marriage, it can also be used as an argument in favor of polygamy.
This is not an abstract thought experiment, but a reality. Jewish law holds the command to “be fruitful and multiply” as binding on all men and women. This leads directly to polygamy in the case where a man’s wife cannot conceive (or has not conceived); Scripture provides case studies as with Elkanah, Peninnah and Hannah, and also that of Abraham (although Hagar remains a concubine rather than a formal wife). The necessity to procreate also leads to the Levirate law in which a childless widow is to be impregnated by her brother-in-law. This law figures in salvation history in the person of Ruth and Boaz; and when the question of the Levirate law is raised to Jesus by the Sadducees, he does not speak against it, in principle, though he does aver that marriage is a thing of this world. Closer to the Anglican homestead, observe the extent to which Henry Rex’s concern for the succession led him to employ, and then reject, the Levirate law; and even briefly, so it is said, to contemplate plural marriage — precisely what the pope accused him of undertaking when he married “Anne of a thousand days.”
On the other side, a negative argument involving procreation is often advanced against same-sex marriage, as a kind of Kantian categorical imperative: that if everyone practiced it the human race would cease to exist. However, the same argument can be advanced against celibacy. Again, this is not merely theoretical, but (in keeping with the understanding of the "first commandment" to multiplication of the human species) forms part of the groundwork for the opprobrium attached to celibacy in mainstream rabbinic Judaism.
Ultimately we owe to the scholastic church the fine argument that the command to procreation is addressed to the species as a whole, not to all individual members of it. This let the celibates off the hook, but the application to family planning has run aground on the shoals of natural law — another example of the fact that arguments can be applied to different concerns with very different results.
In summary, then, it would appear that arguments ought to be weighed on their own merits, not on ancillary or subsidiary possible circumstances. Those represent slopes down which it is not at all necessary to slip.
Tobias Stanislas Haller BSG