represents where the whole concept of marriage came from in the first place.
Actually, marriage historically has more to do with property and creating ties between various groups.
One could argue that it's still more about property and procreation. After all you need a license issued by the State to marry, not so for makin' babies.
In fact there is a whole host of legal/fiscal implications of marriage that are not present in "baby-daddy"/"baby-momma" relationships. (The obvious counter point being child support).
You could even look at marriage as a contract like any other contract. Two men can enter into a business arangement together. Two women can give eachother power of attorney (not at the same time). Two men can buy a house together. Etc.
Why should the State's view on marriage be any different than any other State sanctioned contract.
Notice, I'm talking only about the State, not a Church. Marriage, as a legal institution has nothing to do with the Church. You know when the minister says, "I now pronounce you man and wife"? Means nothing. What's import is signing the marriage license with witnesses.