"In the debate over whether the definition of marriage can be changed, I have not yet heard an answer to the following question: Whence comes the authority to change the definition?"A loaded question calls for a loaded answer, so here's mine:
The authority to change any definition is consensual. Definitions are conceived when consenting adults engage in poetic intercourse, so they change in conjunction with evolving consciousness. That is the basis of semantic evolution.
To see this just think of how the definition of 'gay' has changed over the past century.
So, Mr. McCabe, there's your answer: the authority to change the definition of marriage comes from the seminal font of creativity within each of us.