It's an argument I have made on several occasions -- but never with more brevity than this:
"A frequently overheard slippery slope is:
Gay marriage = Less procreation = Human extinction.
But this slippery slope only works if we all become gay.
What process originating in gay marriage will make me divorce my wife and marry a man?
What process originating in gay marriage will make any appreciable number of straight people turn gay?"
From a commenter on the blog, Volokh Conspiracy.