A magistrate in the state I live in refused to marry a gay couple even though homosexual marriage was just declared legal by the courts. So a couple of questions.
1 If a gay couple ask a conservative preacher to marry them can he site his religious beliefs and say no?
2 If the answer to question 1 is yes should a magistrate be able to say no because of his beliefs?
My only fear now that it is legal is that preachers are going to be forced into marrying them even though it is against his beliefs.
The answer to question 1 is yes, no church is required to marry anyone. Just as the Catholic church can refuse to marry non catholics so can a preacher refuse to marry those who don't share his beliefs.
The answer to 2 is no because a magistrate is not a religious office. It is a secular one and so is a marriage contract. If the magistrate refuses he can be fired for failing to perform the duties of his office.
Your fears are groundless.
Something you might want to research though is that Pope Francis is recommending welcoming gays and divorced catholics back into the RCC. He is setting a precedent here that might be adopted by other churches. He has no power to make them accept gays and perform gay marriage but if gays are welcome in catholic churches and shunned by protestants
it isn't hard to figure out who the winners and losers are going to be.