UCLA law prof and major blogger Eugene Volokh pretty well reams out the argument against gay marriage based on so-called "natural law." Money quote, if you will, but the whole thing is well worth your time:
"God seems to have designed the human body in such a way that the penis, the mouth, and the anus can be used in lots of different ways; why should we infer, simply from the fact that one use (penile-vaginal sex leading to reproduction) is so important, that it's the One True Proper Use of genitalia? Likewise, God has designed humans in a way that allows some of them to be attracted to members of their own sex; even if you believe that this preference isn't innate, but is caused in part by upbringing or by personal choice, it's clear that the possibility of this preference is indeed present in humans (and, as I said, other animals). This too casts doubt on the theory that penises or the sexual act have One True Inherent Purpose or One True Inherent Mode Of Employment."
(Hat tip to Ed Brayton, whose site also has some good comments.)