Any inquiry aimed at discovering the nature of marriage must ultimately arrive at one of two conclusions: Either marriage is something with an absolute nature ordained by God and thus unchangeable or it is an artificial thing, created by human beings on their own authority, and thus changeable according to the whims of whatever members of the human race happen to gain the political power needed to define it for the rest of the species.Yeah that's pretty much it. Jeffrey's conclusion is, naturally, that God created Marriage and we have a duty to preserve the traditional definition of it. Because if we don't than we will have plural marriage and other bad stuff.
The weakness in the argument is that it presupposes that we should legislate based on what God wants. God may very well be the same yesterday, today, and tomorrow (I believe he is) but our mortal and fallible opinions on what God wants are as changeable as the wind. More to the point we live in a multicultural society. Americans practice a multitude of faiths and some practice no faith at all.
What Jeffrey is explicitly asking for is that we set a theocratic law - we rule from the Bible, or to be more precise, his interpretation of the Bible. I personally think that's a very bad precedent, and if that is the strongest argument proponents of marriage discrimination should bring to bear, than I hope we start coming to our senses.