I think in order to tax us as married couples they have to confirm who is acually married and who is not. I think God laid out very clearly what marriage is and who is involved. 1 man, 1 woman, and Him. 3 way covenant.
I also think measures to slow the slide our country is taking towards complete self serving, worldy lusts fulfilling, instant gratification seeking depravity are always a good idea. I heard a preacher once say that " if God doesn't burn this whole country to the ground then he probably owes sodom and gomorrha an apology." Lot of truth in that. I don't care what people do in their bedrooms. That's their business. Marriage however is a different ballgame. Its already been defined. Gods definitions don't change with the tides like mans morality does.
So, your two justifications for this are revenue collection and theocracy?
I see no reason why the government must sanction any marriages. However, it does sanction heterosexual marriages and provides heterosexual married couples with benefits. To not provide these same benefits to homosexual couples is arbitrary and unequal treatment from a non-religious viewpoint.