The word 'marriage' actually means two completely different things. Sometimes we confuse them, and sometimes it's deliberate.
One definition of marriage is a religious sacrament. And churches have the right to decide who they will marry and who not, it's none of the govts. business. Religious people don't realize that the separation of church and state is as much to protect the church from the state as vice versa.
The other definition of marriage is a civil partnership, allowing people to legally co-mingle finances, own property jointly and inherit without probate, be responsible for one another's debts, etc. And this is a CIVIL right, i.e a right of citizenship, which ought to be available to all citizens. Churches have no part in this form of marriage.
Actually some churches these days -will- marry gays. But it's up to them. The thing about civil rights is that one of the most basic principles of our form of democracy is that all citizens have exactly the same rights and privileges. People can't even vote to deny rights to one group. So gays have the right to get married.
But let's wait and see what the Supreme Court says. They will be examining the issue this year.