Haven't sprayed here yet. Thank you for the opportunity. I know I'm a little late but given the importance of the matter and the amount of good cheer it brings to discuss... so, here's this:
PH:
(1) If you allow gay marriage, then there is no reasonable justification to prevent marriage with goats, children, and walruses.
(2) If there's no reasonable justification to prevent marriage with goats, children, and walruses, then something is wrong.
(3) So, if you allow gay marriage, something is wrong.
(4) If anything makes something wrong, then it shouldn't be allowed.
(5) So, gay marriage shouldn't be allowed.
My non-boorish reply:
(1) is false.
An engaging argument that (1) is false:
(1) There is good reason to believe that goats, walruses, et al. are unable to communicate the terms of and subsequently consent to the terms of the marriage contract.
(2) If so, then there is a reasonable justification to prevent their being regarded by law as married.
(3) So, there is a reasonable justification to prevent their being regarded by law as married.
Imagined PH counter-reply:
(1) Marriage is not merely a contractual matter. It includes implicit rules about the natures of the concerned parties--namely, adult male and adult female humans.
(2) If so, then if the law requires such additional rules be enforced, the law should not recognize gay marriage.
(3) The law does.
(4) So, the law should not recognize gay marriage.
Imagined myself's two counter-counter replies:
1. The law doesn't. At least not obviously.
2. Give an argument for the additional rule stuff about human natures that is constitutionally and/or conventionally defensible and we'll talk.