This happens with most highly charged topics, but with the same-sex marriage debate, it seems especially common (making the debate particularly tedious after years of engaging in it): After a few steps setting the mutually understood context, the thread becomes lost in opponents’ eagerness to make their total case. To review the discussion thus far:

• Pragmatist asked why the state shouldn’t encourage monogamous homosexual relationships.
• I replied that I’m not opposed to its doing so, except if done from within the institution of marriage, because modifying the definition of marriage in order to encompass same-sex relationships would undermine the institution, diminishing its ability to encourage stable, monogamous relationships between anybody.
• Pragmatist asked how incorporating homosexual relationships would undermine marriage.
• I replied that three corrosive mechanisms would follow a change in the essence marriage from a relationship between a potentially procreative pair to one between intimate adults:
1. Equal rights abide no arbitrary boundaries. Further changes to the definition of marriage would follow, notably polygamy and intra-family marriages.
2. We won’t abide the government in our bedrooms. The new twist on marriage would be prone to abuse by heterosexuals, who would treat marriage less like a romantic relationship (at least on a temporary basis), and nobody wants the government to be checking to make sure that the relationship is sexual.
3. Too many won’t abide their own children. Muddying the simple definition of marriage as the relationship into which parents ought to enter with each other will make it less effective in creating a cultural expectation that will draw those who might be inclined to shirk their responsibility into stable families.

In short, the argument on the table is that the state should not change marriage in such a way as to include same-sex relationships because doing so will undermine marriage for the three reasons listed. The two possible contrary responses are (A) to argue that equal rights (or some other consideration) make the diminishment of marriage irrelevant to the legitimization of SSM and (B) to argue that none of the three mechanisms will harm the institution significantly. The unfortunate tendency, as an alternative to these two possibilities, is to address each point as if it is intended to stand on its own; the discussion becomes impossible to pin down, because the goal is merely to push the points off the table rather than to address the total argument. It’s the difference between testing the strength of a model and rolling marbles onto a pool table.
In this post, I’ll endeavor to explain why no responses heretofore made in answer to my assertion of the first corrosive mechanism have any effect on my argument as I’ve laid it out thus far.
M. Steven, who appears ultimately to come down on my side on the marriage issue, suggests that the “ban on incestuous relationships is already an arbitrary boundary within the current definition… based on moral grounds.” In other words, its degree of arbitrariness would not change with the introduction of same-sex relationships into marriage. But this is certainly not true: The arguments against incest are, first, that it does physical and psychic harm to begotten children and, second, that it corrupts other interpersonal roles and affects development. Clearly, same-sex marriage eliminates the concern about the first (which I would consider the more decisive) point, and to the extent that the second point remains, it seems to me to have less force when one pictures two brothers, say, rather than a brother and a sister.
Beyond arbitrariness, though, implicit in the SSM cause is the principle that it is unjust for citizens to impose their moral beliefs on others when it comes to something as personal and inalienable as marital rights.
For his part, Pragmatist takes a broader tack, including social, rather than moral, arguments against incest and polygamy (which M. Steven doesn’t address), as well as power dynamics, but I don’t think he appreciates the radical change that would be wrought by the separation of gender from marriage. His suggestion, for example, that both incest and polygamy are socially objectionable because “the power dynamics… lead to the exploitation of women, especially young women” is utterly extinguished if marriage ceases to be defined as an opposite-sex affair. Especially with the case for SSM being made on equal rights grounds, and even more so with its leading edge being in the judiciary (what with “rational basis” tests and other attempts at logical adjudication), a definition of marriage that hinges on individualistic love and mutual care offers no justification for an argument from power dynamics.
This consideration also erases Pragmatist’s attempt to differentiate homosexual relationships from incest and polygamy because “homosexuals are denied the right to marry anyone they love,” as opposed to someone they love (which is supposed all that will be denied of the incestuous and polygamous). I don’t think it would disturb his meaning to edit thus: “homosexuals are denied the right to marry anyone they could constitutively love.” But the distinction is irrelevant to the legal, or even rights-based, arguments of those who desire incestuous relationships. Is the court (or the legislature) to explain to a man that he may not marry his sister because he has other options?
A person who is truly in love, and who has already chosen a compatible mate, could with reason be termed “that-person-sexual.” If, from society’s point of view, marriage isn’t above all about the children who may be born within the family that it creates — that is, if it is only about the adults’ relationship — then the external basis for interfering in the choice disappears. Allowing the someone/anyone distinction would make it valid to argue that barring interracial marriage doesn’t interfere with citizens’ right to marry someone. “But I don’t love any white women,” the Caucasian man might say; “I love this black woman.”
Polygamy reenters the discussion with the distinction between a legal relationship and a legal marriage. I wouldn’t support the criminalization of homosexual relationships. Does Pragmatist support the criminalization of adult incest? Of extramarital affairs and swinging (i.e., non-marital polygamy)? Or consider this one: extra-marital relationships with mutual care and emotional connection, but not sex? One could point out that the polygamist is free to bring others into the relationship but is only allowed an official marriage with a single spouse. Likewise, on could argue that homosexuals are free to form their relationships as they choose; they are only able to enter into marriage according to its definition: with somebody of the opposite sex. (And nobody is forcing them to have civilly recognized marriages.)
The point is that there is a difference between the denial of marriage rights and the denial of relationships. Pragmatist claims never to have “heard of a genetically predisposed polygamist,” but it has seemed a commonplace to me that all men are genetically predisposed thusly. That, indeed, is what makes the following a legitimate concern:

Polygamy undermines the social structure because over time, high-status males will attract multiple partners while low-status men will have no options. Society is unwilling to create a permanent underclass of unmarried males.

Again, though, Pragmatist is apparently unaware of the heterosexist underpinnings of his thought. A man who marries a woman and then marries another man is actually alleviating that underclass. On the other hand, considering that a general takeaway of the reading that I’ve done with respect to sexual orientation is that women’s sexuality is more fluid, it’s not difficult to imagine women marrying each other even though they might have some attraction to men. In such cases, even a two-person marriage contributes to the creation of a single-male underclass. If that concern were proven to have a significant likelihood of fruition, would that allow “discrimination” against homosexuals with respect to marriage?
Pragmatist may attack my “reflexive conservative support of what society has adopted in the past,” but he’s at least as inclined to rely upon precisely that traditional understanding of social implications. In a world with same-sex marriage, old calculations no longer apply. In a world in which procreation is not conceptually intrinsic to marriage, why must marriages be presumed to be sexual at all? Why, for example, couldn’t a mother-daughter pair claim a right to the mutual-care benefits of marriage? It would certainly assist them in jointly raising the daughter’s child from a failed relationship. Or suppose a gay man and a lesbian create a child together and then seek to share parental responsibility, within the structure of marriage, with their gay partners?
And that brings us to mechanism #2.