This case, and this thread, came about because the law was deemed unconstitutional by a judge, not a priest. Railing on religion won't help people see things your way (since people who hate religion are a small minority) and if you look at past history, countries where religion was officially outlawed have an extremely high rate of extraordinarily harsh human rights violations, which is a really mild way of saying that the leaders in those countries didn't/don't give a rat's *** about the people they rule. Stalin was responsible for approximately 50 Million deaths in the USSR and they outlawed religion. China has been well-known for killing female children because they wanted a stronger nation. They also had discouraged religion because the Communist Party thought it posed a danger to Communism. OTOH, countries where religion plays/played a strong role in how they make official policy have a long history of being extremely controlling and tend to try to tell everyone what they can do, when they can do it and how they can do it, along with being their own bag of human rights violations. The difference with this is who they can do it to. Getting married at a courthouse has been allowable for quite a while but it seems that it became a common practice when people A) didn't observe any religion or the couple didn't observe the same one and didn't feel it necessary to require one of them to convert, B) couldn't afford to be married the usual way or, C) they couldn't wait. Marriage is more of a legal proposition now and as such, it's subject to approval/disapproval by voters. People have strong feelings about gays without even going into the whole "should they marry?" and "define civil union vs marriage and what legal ramifications does each carry" area. Some will look at it in terms of how their benefits will be affected at work- a couple, gay or straight, who engages in risky sexual behavior, drug/alcohol abuse or generally bad health will usually cost the company a lot more than a nice, wholesome couple. Most people base their opinions on traditions and it takes a long time to change that. It won't happen overnight and if people don't want it to change, they make rules against it. Democracy is about the majority getting their way, right or wrong. If not, what's the point of voting? We don't live in a pure Democracy, though- this is, or was, a Federal Republic and that has rules about how much control the Federal government can have over the individual states. Each state, republic or commonwealth has its own charter or constitution outlining what its people can & can't do as well as defining the role of that state's government. If the Central government doesn't address an issue or specifically designates it as such, it's a "state's rights" decision.
Our Constitution doesn't forbid religious practice but at the same time, it explicitly forbids the US from having an official one. It's worded the way it is so people can observe any religion they want and if they don't want to, that's OK, too. Almost like saying, "Buddhism- take it, or leave it". What has happened is, the small minority of people who don't observe a religion have decided that when they see ANY mention of God or religion, they're being forced to deal with it.
You wrote that the Christian religion opposes same-sex marriage but some denominations have openly gay clergy. Others have not-so-openly gay clergy. How a celibate priest can council people on marriage is beyond my pay grade to understand.
I don't remember hearing anyone complain about JFK being religious (assuming he observed in his own way, because he has a helluva reputation as a horndog) but he was the first Roman Catholic President and that made some serious waves. I don't remember anyone complaining about Carter praying and being "Born Again". I don't remember anyone having a hard time with Clinton going to various churches and if he/Hillary are actually religious in the traditional sense, they pray and ask for guidance whether they admit it, or not. I never saw Bush I or W telling people that they HAD to observe a religion, yet W gets skewered everywhere for his religious beliefs.
Last time I checked, no state in America was governed by the church, of any kind, with the possible exception of Utah. I was genuinely surprised when CA banned gay marriage. Of all states, I would have thought they were most likely to accept it with open arms.