Gay Marriage Arguments: Blistering Bench Slap or Thrasymachus Redux?

I saw a friend share an article online about Judge Richard Posner blasting away at lawyers charged with defending state statutes regulating marriage in its traditional form.  His posture is not new.  We have seen court after court treat the defenders of traditional marriage as though they had no argument to make outside of prejudice or superstition.  

As I read this last piece, I recalled an exchange from Plato’s Republic in which Thrasymachus challenges Socrates to defeat his highly empirical argument on justice:  “Justice is the interest (or advantage) of the stronger.”  It’s a pretty good argument, really, though Socrates gets the better of the sophist.  At the beginning of the exchange, Thrasymachus tries to prohibit the use of certain arguments by Socrates.  Socrates replies that his brash interlocutor has done something like trying to prevent him from arguing that six times two or three times four is twelve.  

When I look at the public debates over gay marriage, I see something like that dynamic.  The proponents of gay marriage say to the defenders of traditional marriage, “You may not argue from the physical complementarity of the sexes.  Okay, proceed.”  It seems silly to say that you can’t argue from the physical complementarity of the sexes or that such an argument is an obvious non-starter.  Why is it a non-starter?  It seems like a reasonably good argument to me.  You may not be convinced, but it is not a big nothing.  To treat that as some kind of non-argument is to rule out three times four and two times six by way of arriving at twelve.