There is clearly such an argument: "marriage is about biological reproduction."
Of course, consistently applied, that should also argue against marriage of straight couples unable to have children (and probably also all the straight couples uninterested in having children). "No marriage after a hysterectomy" is not a position I've encountered.
No, that argument applies (and was in fact applied) to interracial marriages as well: interracial marriages produce undesirable hybrids which have no racial identity and therefore cannot be integrated into society.
> "No marriage after a hysterectomy" is not a position I've encountered.
Of course you haven't, because no one really takes the reproduction argument seriously. When you dig into it, it's just an obvious smokescreen to cover up the bigotry which is at the heart of all opposition to gay marriage.
If there were an actual argument against gay marriage, any demonstrable harm that comes from it, don't you think that the Right would be shouting it from the rooftops? But they aren't. All they're shouting from the rooftops is how horrible it is to ram social change down people's throats. The fact that this is the best argument they can muster is proof that sometimes you have no choice but to ram social change down people's throats, because some people are simply impervious to reason.
My point was not that it's a great argument, devastating to The Gay Agenda. My point was that it is an argument that does not equally apply to interracial marriage. The fact that someone could stack additional assumptions on top of that argument to get an argument that does apply to interracial marriage is irrelevant - you've built a different argument.
And it's absolutely an argument I have heard. Handwavy denial of its existence to serve your rhetoric is poor form. Addressing the argument directly should be easy enough; I touched on why I think it's a poor argument. I agree that those who make it don't consistently apply it - I raised that point explicitly.
Edited to add: To elaborate on why the argument you mentioned against interracial marriage is a different argument in the sense that is important here, one could easily believe that marriage is about biological reproduction, but believe that interracial "hybrids" are not a bad thing, and thus not be inconsistent in accepting interracial marriage but rejecting gay marriage on these grounds.
> My point was that it is an argument that does not equally apply to interracial marriage
I didn't say it could be applied equally, I said it could be applied just as well. Those don't mean quite the same thing. To review, the argument as you presented it was:
"marriage is about biological reproduction"
That exact argument (in almost those exact words) can be and in fact was applied to interracial marriage back when that was still a thing. Look at e.g.:
The first quote is "They cannot possibly have any progeny, and such a fact sufficiently justifies [not allowing their marriage]." (State v. Jackson. Missouri, 1883)
The rhetorical force of your initial statement came from an accusation that one could not be consistent in denouncing gay marriage while accepting interracial marriage. Once you add additional assumptions, or rely on claims of fact that are true in the one case but false in the other, that does not hold. "Your argument sounds a tiny bit like something that was used to argue for something wrong" is not a good refutation.
As an aside, it looks like the quote is misleadingly truncated in a way that substantively changed the meaning. That said, the full version is still flagrantly factually inaccurate, so the reasoning above remains unchanged.
Of course, consistently applied, that should also argue against marriage of straight couples unable to have children (and probably also all the straight couples uninterested in having children). "No marriage after a hysterectomy" is not a position I've encountered.