Personally, I don't think that religion is really the main factor here.

People always just pick those parts of religion that they like and ignore the parts they don't like. Think of the biblical prohibition of interest, or Jesus - Jesus and his teachings are downright communist/socialist, the most  remarkable thing about the first christian communities were that they were extremely communistic. And yet, with the USA being one of the most Jesus-adoring countries in the world, there's probably no country on earth where communism/socialism has a worse public image. Quite funny!

So I think it's rather that today's average citizens are simply at least still slightly homophobic, and they just argue with religious teachings when these seem to confirm the rightness of their own homophobic opinions.

But btw, the discussion reminded me of a vanity card that Chuck Lorre showed at the end of a recent The Big Bang theory episode:

"I believe that inherent within the God-given right to the pursuit of happiness, is the equally God-given right to the pursuit of unhappiness.
That is why I support gay marriage."

BTW, can anyone who knows gay people who have already married or are considering to do so tell me what their actual motives for marrying are? Because I wonder if it's mainly for rational/economic reasons, or rather because they like the archaic, nowadays often kitschy ritual?

I think you're missing the point.  If religions want marriage to be a sacred institution, let it be so.  Whether the term is civil unions or some other term, use it to describe a civil union, which is what a marriage is outside of sans religion.

On the contrary, I think it's driven more by people who aren't frothing extremists and don't go around protesting military funerals. There are likely a lot more people who see the LGBT community as a bunch of mental defectives and fetishists, but just don't talk about their views much.

I'm not missing the point at all. My point is that your point is absurd. It's not simply about how the term is used. It's an extension of the bigotry that Christians show others. It doesn't begin and end with homosexuals and marriage. They should not be awarded anything. Marriage is NOT a religious concept or term. It did not begin as, nor is it, THEIR sacred institution. Christians want you to think that it is. The only reason to call it anything other than marriage is because religious bigots can't stand the idea that they are not, in some way, denying people something if those people don't follow the rules that these Christians have chosen for themselves. They want a term and concept which predates their religion to be legally recognized as a Christian idea. If they don't like it then THEY can be the ones who choose a new term. Not force everyone else to accept the term as theirs when it never was to begin with. Besides, this is ultimately about striking down homosexuality. They don't bother trying to protest non-religious straight marriages which should be considered just as blasphemous to them if they think what they're trying to uphold is some kind of Christian born value or idea that only Christians should be allowed to engage in. This is much less about the ownership of a term and much more about trying to shaft a particular group of people that Christians don't like with every chance they try to give themselves.

Not only same sex marriage, also science and logic.

I don't know what your trying to say. Are you saying Christians are extremists?

Part of the problem with your title is the word extremist being associated with Christians, that is liberal hate speech.  But Christians do have a problem with same sex marriage and its not extreme to for them to have that belief.  The idea that a society works best under certain conditions is not an extreme point of view.  

I personally don't believe that gay marriage should be illegal but I won't condem those that believe otherwise because for literally hundreds of years marriage between man and woman has helped build great societies.  Now if happily married gay couples can contribute to that greatness we will have to see.  Will they adopt children in need of parents as research shows children with two parents do much better than single parent children.  And when their children go to school and find their teacher is talking to them about transgender and sexuality in k-5 schools will they speak out against it because kids that age have no business discussing this issues.  

The truth is the gay community has been clawing and scratching to be a part of something that when they finally get it may realize its a lot harder than they thought.  But hey maybe it will bring some of the gay community around to a more conservative point of view once they realize that liberals don't really care about them and their life, and to liberals the gay community is just another project to change and then leave in ruin.

I really don't see how any part of this makes sense. How exactly would liberals leave the gay community "in ruin"?

Lots of people care. Just because your view is now politically dominant does not make those people who hold that homosexuality is abhorrent 'extremists', especially as it is the majority view worldwide and has been the only viewpoint for much of recorded history. The liberal elite in our society have taken the word 'extremist' and redefined it as anything standing in the way of their dismantling the fundamental structures upon which our society is built.