My choice to oppose legalising gay marriage is based on my understanding of the God who made us telling me that it is a contamination of his design, and that it is harmful. Contrary to the popular opinion of our society, the bible does not consider homosexuality to be the unforgivable sin, or gay people to be irredeemable, but it does quite clearly state that homosexual sex is a sin. [See Leviticus 18:22, 20:13, Romans 1:26-27, 1 Corinthians 6:9-20, 1 Timothy 1:8-11]
I also believe that homosexuality is not a natural part of a person but a distortion of the true person. This is the case also with marriage, that it is designed to be heterosexual monogamous and life-long and that anything else is a distortion or a contamination of the ideal. Even a small distortion or contamination is a serious issue. Consider that 99% water and 1% sewage is a small distortion of the purity of the whole, but I'm not going to drink it, or advocate that anyone else should, and if someone wants to pass a law that all the water should be held to that standard, I will oppose it in favour of a better definition of what is acceptably called drinking water.
For the most part, Christians in this issue are (whether you agree with them or not) simply doing what we all must in a democratic society and making their opinion heard, which is the same right (and responsibility) each person has in a democratic system. Remembering that in a democracy, a vocal minority can win over a silent majority because the aim is to give the power to the majority, but really only those who voice an opinion or cast a vote get counted.
But there is a question that doesn't seem to get put forward, and that is one of living at peace. The scripture says "If it is possible, as far as it depends on you, live at peace with everyone." (Romans 12:18) which means that the question has to be asked: is this issue (which in one sense is more a legal one than a spiritual one) so vital that it is worth sacrificing that peace over? Many people would say yes.
If a girl tells her brother he can't have ice cream the brother would rightly say "by whose authority?" he might say his sister hates him because she won't let him have what he wants. If there is a parent who says it, and they say it’s because the child is lactose intolerant the boy might still call it hateful, but it would be out of love that the rule was made. If the parent is absent and the girl knows the rule, then she should tell her brother what their parent said. If the brother ignores her, then she has at least done what she believed to be right. If she truly loves her brother, she will be more insistent.
It is important to understand the Christian understanding of true and complete freedom which is not simply about the ability to do whatever you want whenever you want to, but rather the ability to choose to do what is right without being dictated to by your own desires or needs.
Consider what Paul says in 1 Corinthians 8 about eating meat that has been sacrificed to idols. He essentially says that it is not a sin to eat that meat, because the idol is really just a chunk of nicely shaped wood or metal, but for the sake of another person's wellbeing he will choose to become a vegetarian rather than cause them a spiritual dilemma.
Is it not better to willingly lay aside our right to be heard as members of a democratic society on this issue, in the interest of promoting more helpful dialogue, than to be the vocal minority who make a war of an issue that is not really a spiritual one, and be the cause of more hurt?
It seems to me that the point of what Paul is saying both in this particular example and in several other places is that Christian freedom encompasses the ability to choose to willingly take the short end of the stick particularly in the interest of positive relationships with fellow believers and positively reinforcing others' (not necessarily limited to believers) connection with Christ.
The truth is that I have heard of a number of people who are suggesting that what Christians should do is relinquish the names of things we feel have been co-opted and corrupted by being mixed too much with secular society like marriage (also Easter and Christmas) and rename them to distinguish them in a truly Christian sense. To let society do what it likes with marriage and choose instead to have the equivalent of a civil union or a de-facto relationship with a spiritual covenant rather than what the laws of our government call a marriage. The point, as I understand it, being to wilfully choose the legal standing that gay people are trying to reject as inadequate. To lay aside legal rights and entitlements that others fight for and accept the short end of the stick in an effort to come back to the ideal and to stand apart as holy to the Lord. To distinguish the Christian sexuality and marriage as apart from the social and legal ideas of it. This is something I think might be worth discussing further.