your basic premise if flawed though - marriage is a legal ceremony , not one that is only the province of the church.
I am an aetheist (well , that isnt quite true, I believe in a higher power, but not in organised religion) , my wife agnostic, but we still had a civil wedding ceremony and are legally maried with no involvement of the church/god at all ( I have absolutely zero interest in being 'blessed by god' - I would hope that a supreme diety has better things to do than worry about blessing man or indeed being worshipped)
why should a gay couple be denied the same right ?