Of all of mankind's greatest assets, we know of only three things that preceded marriage: Sex, Love, and fire... and we aren't 100% sure about fire.
Just think about this for a moment. Everything that makes mankind great: medicine, science, math, philosophy, psychology, finance, politics, religion, war, THE VERY CONCEPT OF FAMILY!!... EVERYTHING in all of human history, all of it was created after marriage. Marriage preceded the invention of the wheel, writing, the pyramids, the Trojan war, the great flood, Egypt, Greece, Rome, Stonehenge, the Aztec and Mayan pyramids. All of them came after marriage. The human presence all over the world came after our predecessors invented the marriage covenant. Really. There were no humans in America. There were no humans in Europe. No humans in Asia until marriage was created.
Marriage literally was the very first act of society. When man and woman began to vow to live together till death and care for their common young, society began. Nothing created by society preceded that. Nothing.
Marriage literally is the beginning of the greatness of mankind. It is perhaps our greatest invention because it created society itself.
And the definition of marriage, the definition that those early people created, has remained unchanged for tens of thousands of years. Perhaps longer. It has always been between a man and a woman.
Now we have to change it? You are asking us to change the very definition of mankind. Mankind is a social creature. Social life lies at the heart of mankind. Society began with marriage. If we change the definition of marriage, we change society itself. We change all of humanity. 97% of humanity has to change for 3% of the population.
That is what you are expecting.
THIS is why we are against same sex marriage.