Bottom line (as I said in t'other thread).
We have only progressed to gay marriage, because there was already such a thing as straight marriage. Therefore, straight marriage was the start of the slippery slope to gay marriage, and is therefore responsible for any further changes. Abolish straight marriage, and all the concerns he has about people marrying toasters will vanish, because "marriage" will not exist.
Makes as much sense as any of their bonkers objections.