Refer to it as “Holy Matrimony” and the Church remains exempt.
The government can tell people WHO they must serve, but they can't tell people WHAT they must provide. If the church only has one ritual, then that's all they do. One ritual. The gays will just have to like the man/woman holy matrimony (otherwise it's not holy) or maybe find somewhere that has more than one specific ritual.
The homos have to be offered a holy union between only a man and a woman in the eyes of God ritual, though, or it'll be illegal. (They won't be married in the eyes of God either, because the ritual wasn't given for a man/man/it or whatever. It will be given to the air floating around in the room, because there's no bride and groom present. )