I understand why Catholics, with their notion that marriage is a sacrament, would wish to refuse to conduct one for same-sex couples.
But, as an evangelical Christian, my concern is less whether I should be compelled to do so than why on earth any same-sex couple would want me to. Church weddings have to be conducted according to the rites and ceremonies of the Church in question.
Jumping over the inconvenience that no churches have (or have ever had) any rite or ceremony for such circumstances, it is without question that in any evangelical, or reformed Church, a wedding includes a sermon on a topic of the minister's choice.
Were I ever to be obliged by the civil authorities to conduct such a wedding, my sermon would include a forthright exposition of the Bible's teaching on sodomy and the evils of laws made by godless governments.