This is a bit of a departure from our usual analysis of particular arguments in the media, but because these arguments are fairly common and because we've been hashing these issues out in the comments to the earlier post "5,000 Years," I thought I'd try to synthesize the analysis of the argument as I see it.
Is there a non-question begging (secular) argument for the following claim?
C: Same-sex relationships cannot be considered "marriage."
Setting aside certain circular arguments about tradition (like Rick Warren's which was originally being commented on), the best argument seems to rest on the premise:
P: A necessary condition of marriage is the biological possibility of procreation.
Here biological possibility has to be understood as satisfying the counter-factual condition:
BP: If the functional organs of procreation are working in a species typical way, procreation would be biologically possible.
This condition is meant to include infertile and older couples within the scope of the condition, while still excluding same sex couples. I am not, of course, endorsing this exclusion: the question is whether a good argument can be constructed for C, as a matter of logic, that could justify arguments against same-sex marriage. I am tempted to claim that there cannot be any such argument after considering the various arguments.
Because the argument is trading in essences and definitions it would seem to be deductive: That is, it argues for the impossibility of same-sex marriage by appeal to a definition/essence. It has the form of:
1. X is a necessary condition of Y.
2. Necessarily, Z does not have X.
3. Therefore, necessarily, Z is not Y.
Triangles must have straight sides. Necessarily, Circles do not have straight sides. Therefore, necessarily, circles are not Triangles. Or, Nougaty filling is a necessary condition of being a Three Musketeers bar. Necessarily, Toffee does not have a nougaty filling. Therefore, necessarily, toffee is not a Three Musketeer's bar.
As such this looks like a valid deductive argument. But, a critic might wonder whether P understood in the light of BP really says anything more the following implicit premise.
IP: Only heterosexual couples can be married.
If this is so, then the argument might reasonably be accused of begging the question. But determining when the question is begged needs to be handled carefully, since a begged question can always be resolved by appealing to some further argument that independently justifies the problematic premise.
So, the question then becomes, what independent reason can be provided for P/BP? What sort of "warrant" can be given to claim that marriage has an essential link to the biological possibility of procreation?
In the comments, we identified two distinct strategies:
a) Appeal to tradition/Generalization from past practices–this can range from some sort of descriptive anthropological claim, to some sort of generalization to a normative claim, or a most often a simple stipulation on the basis of past stipulation.
b) Appeal to social function of marriage as defining its essence (coupled with an argument that marriage is the best means for attaining the relevant goals).
It seems to me that (a) either begs the question if it appeals to tradition, or, fails to attain the universality that seems to be needed to underwrite P/BP (at most the generalization can show is that marriage has been understood to have an essential connection to the possibility of procreation, not that this is essential for it. And counter-examples are too many to make the universalization possible (old people getting married, infertile couples etc. And it's no good saying that marriage has just been socially constructed this way, since we are aiming for an essential connection.)
The appeal to tradition seems to me to beg the question insofar as it takes the following form:
1. Marriage has been understood (in the past) to require P/BP.
2. Therefore, P/BP
[I probably don't have the logic right here. I'm realizing as I write that I'm not quite clear on how "appeals to tradition" really work, though I think that they are typically bad arguments. I guess they're a sort of temporally dispersed ad populum.]
Even if we can avoid begging the question here, the problem with this argument is that insofar as it appeals to people's opinions about marriage, it relies on a convention, which doesn't seem to be able to underwrite a claim about essence. At most it underwrites a sort of stipulation which isn't adequate to the purposes of this argument.
The strategy of (b) fails for slightly different reasons. The argument seems to run something like this:
1. Marriages provide for stable procreative units.
2. Society has an interest in stable procreative units.
3. Therefore, Society has an interest in recognizing marriages that are means to stable procreative unit
4. Therefore, Society does not have an interest in recognizing relationships as marriages that are not means to stable procreative units.
This is a fine argument as it stands, but it doesn't get close to showing that there is some sort of essential incoherence in the notion of a marriage for some other purpose (adoptive child-rearing for example). It needs to conclude something much stronger than this, something that would suggest that recognizing same-sex marriages is incoherent, since it aims at establishing P/BP. At most it has shown that from the perspective of society, whether there are same-sex marriages or not is a matter of ambivalence. I think typically the argument seems to succeed because it trades the elision of biological function and social function. A little dose of evolution seems to suggest that this necessity is somehow a species necessity, but I think those arguments are pretty empty. (That is, I don't think we can deduce the "right" social institutions from biology, though I certainly grant that there are lots of ways in which biological truths affect which institutions are desirable and which not).
I'm not at all sure about much of this, and I'm sure there are strategies that I've missed. And certainly it is always open to the arguer to appeal to the Bible or personal communications with God to justify P/BP. But, as far as I can see, I cannot find a viable strategy to make the argument non-question begging. The problem is that the opponent of same-sex marriage must offer a very very strong argument that concludes impossibility if they want to trade on an putative "essence" of marriage. But, the arguments that would establish this putative "essence" of marriage seem to be either too weak to do so, or end up begging the question. The problem is that the "tradition" of heterosexual marriage might have arisen because it was socially useful (and perhaps still is) for managing procreation and the family, but that does not enail the necessary link between marriage and procreation. If this is so, then the whole strategy needs to be rethought, as it is doomed to failure. But, maybe I'm missing something obvious.