Now before I start getting into the topic of the post I would like to acknowledge that this doesn't apply to all Christians, and to those who embrace humanist values, I am letting you know we are on the same side.
When I was younger I was as guilty as any of this bigotry, not because anyone taught me to be, but because I read it in the Bible. I was probably still in primary school the first time I read the book of Romans in the new testament, and before I had even finished the first chapter this verse shows up. Romans 1:27 - "In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion." This is not taken out of context or from a strange translation of the Bible, it is exactly as you read it from the New International Version. Some other translations put it even more harshly than that. This is not an isolated incident in the Bible of this kind of bigotry, in fact it stems all the way back to the pentateuch, where homosexuality is called an abomination, and by mosaic law homosexuals must be stoned to death.
Were it not for this intolerant hatred contained in the Bible that I read as a child, I probably would have spent my teenage years with a rather different set of moral values. Perhaps I would have adopted a set of values closer to my current ones. Or perhaps I would have been more influenced by the teachings attributed to Jesus in the Gospels, with things like 'Let him who goes without sin cast the first stone' and other things like that. I imagine that most of the people of the anti-gay movement would have a similar experience would it not have been for the bigotry taught to them by the Bible. I cannot for the life of me think of any other reason on earth why some people would be so intolerant towards another human being. There certainly aren't any logical, rational reasons for people to hate gays. We live in an overpopulated world, and the fact that two men, or two women cannot reproduce (without modern methods of fertilization) really doesn't matter.
That leads me to my next point, infertility. One of the many appalling arguments against gay marriage commonly used by conservatives is that they cannot reproduce. One must then ask the question about infertile straight people, If gay people aren't allowed to get married purely because they cannot reproduce then the same reasoning must be applied to infertile couples. As you can see this argument falls so short of its intended goal that I feel pity rather than disdain for its supporters.
Every human being has rights, whether Jew, muslim, black, white, gay, straight or anything else. One thing you do not have the right to do though, is to deny others their rights.