Churches have the right to decide who they will and won't marry. Not the question.
Why is a totally baseless opinion, that even the adherers won't even try to defend, and is based on their imagined deity, a valid argument in a secular country?
If they want to huddle amongst themselves in their church and agree the gays can't get married there, fine. That's theirs.
But out in reality, when you want to deny someone their rights, people are going to ask you why and expect you to have a reason.