Government DOES NOT "grant" that which is marriage. It RECOGNIZES marriage for what it is, what it has always been. And it MUST recognize it because of all the social externalities which result from this union.
There is very little, if anything, that government does to manage marriage. People can get married any way they want, with any kind of ceremony, or none at all, and conduct the entire length of their marriage as they see fit, without ANY involvement of government.
That a man and a woman are deemed to "become one flesh" when they wed is simply common tradition and religious conviction, if not an actual reality in many aspects. That they are TREATED as one flesh, by government or any outside party, shouldn't surprise anyone. This "one-flesh" way in which they are treated is expressed in the societal and governmental policies toward marriage, joint-filing of income tax (as one entity,) the right against testifying against your spouse in court (the 5th Amendment, testifying against YOURSELF), etc.
And then people started demanding government money and privileges to go with that recognition. Never mind that Article 1, Section 8 of the document that 21st century conservatives pretend to have reverence for doesn't grant that power to the feds. You wanted big brother's blessing...you got it. It's time to pay the check. Stop complaining now that he's going to do the same favors for the Island of Misfit Toys.