Let me also state that there are many people in the more liberal parts of the church who would disagree with me on what I am saying about what Christians believe, so I will say upfront that I am speaking from the perspective of what most people would call a conservative or fundamentalist position, where fundamentalist means that I take the text of the bible seriously and do not dismiss certain parts of it because my culture disagrees with them. I believe that the bible is the word of God and is necessarily the foundation of Christian belief.
It is not my intention to insult people - I don't push my beliefs in the faces of anyone, but I do feel the need to respond when someone says something about what I believe, and I do think that it is important to try to clearly explain my beliefs in an open forum so that anyone who wants to can try to understand them, and anyone who doesn’t want to know doesn’t have to pay attention to them.
What I was suggesting is that if the Christian definition of marriage is what we use [heterosexual, lifelong, monogamous, covenanted spiritual union] this is generally not what gay people are campaigning for. My observation has been that Christians are not looking to intentionally deny rights to gay people, but rather desire to preserve a sacred institution which has, in their view, already been heavily secularised and badly damaged by misuse and misunderstanding.
If you ask a Christian whether they think gay people should have the right to get married, their answer might well be that if it is gay, then it isn't marriage – it is something else, like the sandwich with no filling isn't a sandwich: it’s bread. We would feel silly campaigning for bread's right to be legally declared a sandwich equally without prejudice about whether it has a filling or not. There is no agenda hidden here to stop you eating the bread without a filling, only that it is silly to insist that it is a sandwich.
What we have is a legal question tied up with a spiritual one – in that we have a governmental institution with legal ramifications based upon a spiritual institution with social ramifications where both carry the same name.
I don't think anyone has any specific issue with a gay couple having the right to publicly declare their love before witnesses or their decision to live together, even for them to have a ceremony in which they pledge their fidelity to each other. In truth, I don't think anyone currently has any kind of legal grounds to stop them doing precisely that anyway.
It has been my observation that in most cases the significant question seems to be that gay couples want to be legally recognised as legitimate with equal rights as a similar married couple, so "unionage" (an idea a friend of mine proposed as a legally recognised union which is in every way identical to a legal marriage but without any of the religious names or connections attached) or something similar would probably not cause a problem for most of the folks who are protesting.
It has however, been suggested by some others that the idea of “unionage” is akin to insisting that the black people are allowed to ride the bus so long as they sit in the back half, away from the decent folk. That may be a fair assessment. I think more discussion on that question is needed.
The position of the Christians who oppose gay marriage is generally centred on their concept of marriage as a scriptural thing which, in their view, predates everything besides the existence of the first human beings, whose first significant action was marriage instituted by God himself (you can see this in Genesis, specifically Genesis 2:18-25, if you want to look it up).
People like to point to the various examples of people in the bible who did not follow the pattern of marriage as God set it up in order to argue that Christians are being hypocritical by not enforcing polygamy or other similar things, but they fail to take into account that the Bible does not record that God told them to behave like that. It should rather be seen as a number of people who failed to follow God’s design for marriage. Consider Abraham, for example, who had one wife as he should, and then when God promised him a son, he chose to have sex with his wife’s servant to make God’s miracle happen. This was certainly not what God had in mind, as is made clear in the following text, and caused him all kinds of trouble later on.
Many Christians see the question of legalising gay marriage as one more step away from the ideal which has already suffered many similar steps away. The atrociously high divorce rate is a testament to the fact that what we're doing is not working positively. One more step in the wrong direction can't make things better. If you want a positive change you need to do something different, not continue the trend.
I don't hate gay people, and I don't think they should be treated as second-class or sub-human any more than I think that an alcoholic, a glutton or a philanderer should. Neither do I believe that they should be pressured to conform to Christianity any more than I think I should be pressured to conform to atheism (which is actually frequently my experience). I just believe that they (like pretty much everyone) have need of some emotional and spiritual growth.