On what basis could they decline if the government says it is a constitutional right? Do you think someone's constitutional rights can be denied just because someone else doesn't like what they do?
You are asserting if both parties have rights, those rights will conflict --- a couple's right to marry and the clergy's right to deny presiding over any wedding. Why do you believe the couple's right will win? As it is, now, the clergy can deny marrying anyone for any reason they want. How will SSM suddenly cancel out the clergy's freedom to practice his/her religion?
Your basic premise is that legalizing SSM will obliterate the Establishment Clause. I don't see any evidence that would happen.