I recently started reading Balkinization (a left-leaning legal commentary blog) to complement the Volokh Conspiracy. I would highly recommend both of them, by the way, for thought provoking articles.
I ran across an article from a few weeks ago by Andrew Koppelman responding to an article by Robert George on Mirror of Justice (a Catholic legal blog). These articles are part of what seems to be an ongoing argument about homosexual marriage, particularly in terms of its relation to polyamorous marriage.
Robert George's point seems to be that accepting the arguments in favor of homosexual marriage would necessarily mean accepting the arguments in favor of polyamorous marriage. The tactic, of course, is to convince people who think that polygamy would be a horrible outcome that homosexual marriage must also be a horrible thing.
While there are certainly many shared arguments between the homosexual movement and the polyamorous movement (freedom of personal choice, non-discrimination based on sexual preference, etc), an assertion that they are identical is, frankly, ridiculous.
To be clear, I have no strong reason to be against a truly polyamorous relationship, or even a polyamorous marriage. However, there are certainly arguments against polyamorous relationships that could not be applied to homosexual marriage: the difficulty of applying existing civil laws meant for a relationship of two people to a group of many people, the possibility of people abusing marriage laws in large groups for tax purposes, etc. Those, and other legitimate concerns with polyamorous marriage, cannot be reasonably applied to the homosexual movement.
However, Professor George arrives at his conclusion by an even more ridiculous path. George says:
One of the many propositions we will set forth and defend (and defy Andy and others who take his line to defeat) is that the redefinition of marriage to remove the element of sexual complementarity perforce eliminates any ground of principle for supposing that marriage is the union of two persons, as opposed to the union of three or more in a polyamorous sexual partnership.What I take Professor George to mean by "the element of sexual complementarity" is the ability to reproduce. (Koppelman, who knows George better, also takes it this way.)
This is an incredibly weak argument. Even assuming that the ability to reproduce is a legitimate basis on which to restrict marriage (in which case it would preclude homosexual marriage), it would not restrict polyamorous marriage. In fact, the reproduction-interest would seem to promote heterosexual polyamorous marriage. Even without any statistics, I feel safe predicting that a polyamorous sexual relationship would have a higher likelihood of producing offspring than a monogamous one.
So, no, removing "the element of sexual complementarity" does not affect, at all, the "principle for supposing that marriage is the union of two persons, as opposed to the union of three or more in a polyamorous sexual partnership."
As an aside: there are many, many problems with the reproduction justification for marriage. First of all, it supposes that reproduction is a legitimate societal interest (does the planet have a shortage of people?). And if we accept that rationale for marriage, it would ban the use of contraceptives in a marital relationship; it would ban marriage for women past ages of fertility, or for infertile people of any gender; it would require the divorce of couples who ceased sexual relations...
The reproduction rationale is irrational to begin with, but it in no way has any bearing on the connection between homosexuality and polyamority.