That is really what I think.
I personally believe that the institution of marriage is intended to provide a stable family for the purpose of raising children. To me, that implies a heterosexual marriage.
The compact that is made is between the two people and God. It is none of the government's business.
In that sense, I suppose I could support a gay marriage or a polyamorous marriage. I do not -- but I still don't think it is the government's business. It is God's business.
You're wrong. When people are married, there are all sorts of legal implications concerning property, financial obligations, and so on. Even within the context of Biblical marriage there were legal/governmental implications.