Before Vrai banned Sappy, my point was to show that marriage can be a non-religious thing. Especially in terms of registering one's relationship with the government, it is non-religious. So, the claims that religion is the sole guiding principle for why one would be opposed to changing the definition of "marriage" from "one man, one woman, of sufficient age, and of sufficient distance between their blood relationship" that it has been since the beginning of the United States is wrong - it is not a religious thing.
To register with the government is to attain the petty "perqs" one gets, even at the expense of some tax advantages for high dual earners. The government provides those perqs due to the stabilizing nature of the relationship.
I was working that angle along with the claim that only religion gets to define what is allowed to be called "marriage." This was why I was asking if three brothers could "marry" by the new definition, or if there are still restrictions (like age, familial relationships, number of parties in the "marriage", etc.). By showing that he, too, would restrict these things he was demonstrating that there are restrictions on what "marriage" actually is, and the argument is about what those limitations should be, not a religious argument. It would show that he, too, wants to limit what "marriage" is to HIS definition.
By showing a lack of religious requirement, lack of complete openness on the definition, and lack of advantage to society at large for anything but "traditional" marriage, I would show that his position (from every angle) was wrong on why people (who think) are opposed to the change of the definition of marriage, thus ending his relentless degrading characterization of Christianity as some kind of boogeyman. It might have even opened his eyes to recognition of a broader scope of humanity than SJW vs. everyone, or right vs. left.