In my humble opinion the Christian Bible defines the word "marriage", and most religions also have their own version of that WORD which means whatever they say it does.
The Christian Bible pretty clearly say a man and a woman, and limits their religiously sanctioned blessing for that union to heterosexual couples, however just as the same word is used by other religions, it's also used by the state to describe the associated MONETARY AGREEMENT that the state issues, documents and enforces. In many places the state's requirements for entering their financial contract have nothing to do with religion, God, or specific sexuality requirements.
In fact the word marriage used in the Bible has not applied to people from different religious backgrounds ever. A Catholic person has never been able to get married in a Hindu ceremony or even by a Baptist or Protestant Minister, the marriage would not be considered valid. Buddhists have their version that they too translate into the word marriage, but it isn't religiously acceptable to Christians.
In the US you won't be considered married by the government's definition, unless you sign their non-religious financial contract, which has NOTHING to do with your religious marriage agreement you can also get if you so choose, from the religious institution of your choosing. Most religious people get both and many Americans only use the state contract.
The problem with the word marriage is that many religious people get horrifically offended when it's used to describe a union between people who aren't "a man and a woman". Most have never been offended that Hindu, Buddhists, and Muslims all over the world, including the US have been using that word to describe their unions. They acknowledge the fact that those people are using the word to describe a different type of union. They know the state issued marriage contract has NOTHING to do with their religion, but some how they view it quite differently and are offended.
The word Marriage didn't exist before the 14th century, it's not something any religion, even Christianity claims came out of the mouth of God. It's a man made verbal expression created to describe unions between Christian men and women. Today in the US it is used to describe obligatory agreements between two people.
I think it's time for religious people to face the fact that they can't stop other people from using their sacred word to describe something they disapprove of. Most are not offended when the word is used to describe unions that don't fit their religious views, they are only upset now because they disapprove of homosexual behavior.
If you are offended by Gay people getting married, get over it, I assure you their use of the same word you think is sacred will not diminish or change your own religious commitments. In fact I think perhaps you should feel honored, since they have so much respect for your institution of Matrimony that they are trying to copy it the best they can.