participating in a gay wedding ceremony is very much against many people's reasonable interpretation of religious commandments.
Not, it's a blatant refusal to obey Jesus' extremely clear commandment:
"(...) whosoever shall smite thee on thy right cheek, turn to him the other also. And if any man will sue thee at the law, and take away thy coat, let him have thy cloke also. And whosoever shall compel thee to go a mile, go with him twain. Give to him that asketh thee, and from him that would borrow of thee turn not thou away. (...) Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you; (...) And if ye salute your brethren only, what do ye more than others? do not even the publicans so?" (Matthew 5:39-47)
In other words, for the analogy-impaired, I'll rephrase the above:
"And whosoever shall compel thee to bake a cake, bake him twain."
"And whosoever shall compel thee to arrange them a bouquet, arrange them twain."
"And whosoever shall compel thee to take 100 photos, take 200."
And so on, and so forth.
Pretty clear, eh? Those refusing service to "sinners" aren't only breaking the law, and morals, and ethics, they are also themselves sinning against their God's will.