It is a separation of church and state thing. No one is asking churches to change who they allow to marry. That's not the same as having legal rights to marriage.

Marriage has two aspects: a legal one and a religious one. So yes, the government HAS TO GO THERE because it has already been there since the Magna Carta. At the center of this is Common Law and jurisprudence.

I'll say this again so you don't ask: I am only going to give a brief history and logic of legal (civic) marriage. This will have no implications to the history of who was allow to marry inside a church. This is about the part you go through whether you call a vicar or just a sea captain (in my case, my wife and I were married by her best friend from grade school -- who is also the state prosecutor of New Hampshire).

Civic marriage has been fundamental to Common Law. The US federal government and all US states except Louisiana follow Common Law. Common Law is an ever-growing set of legal precedents that can change both by legislature and by judiciary process. Much of the marriage part of it is older than the Constitution or the US as a nation -- we're talking English Common Law. Just as a contract signed before the US broke off from the UK can still be binding, so too our marriage laws have precedents going way back.

So what? That means civic marriage is thoroughly tested in every court. Civic marriage is sacrosanct, legally speaking. Once X marries Y, no hospital can say "no, X's parents get to call the shots about X's pull-the-plug request." Y calls the shot. If the hospital disobeys Y as a representative for X, they'll get sued and lose.

Any para-marriage act is untested. It's not clear how much a hospital or even a bank has to respect a partner in a civil union. It doesn't have the same divorce procedures, it varies even between the few states that have them, and it definitely doesn't get you its own standard deduction on your tax forms.

I mentioned Louisiana earlier, as they are a Canon Law state. This means their state laws evolved from the Napoleonic Code. Their courts have almost none of the jurisprudence powers. This means the judge cannot interpret which precedent is at the root of a decision -- it's only how the law was written that matters.

A Canon Law nation or state can make up a para-marriage and it will have all legal bindings. France had this for about a decade with the PACS. A Common Law state can make up a new para-marriage contract type, but it can be shot down as legally invalid by a high enough court.

The most recent precedent for the solidity of civic marriage is Loving vs Virginia, 1967. This Supreme Court decision tossed out all anti-miscegenation laws, not just Virginia's: it was no longer legal to bar a checkerboard couple from getting married.

Let me boil this down: gay marriage is civic marriage, or no marriage contract is meaningful. There is nothing in the earliest legal assessment of marriage that determined who could be the parties involved, with the exception of blood relatives. It was founded to determine lineage of property, and now determines how easily one other person could ruin your credit rating.

Marriage provides an amazing set of legal features. Don't let random churches steal that value.