Many Christians in the gay marriage debate like to talk about historical marriage. By this, of course, they mean that marriage is between a man and a woman and has never historically been between 2 men or 2 women.
This point must be conceded. However, are we bound by the traditional norms and mores?
Historically, marriages were not done out of a sense of love or romance but were arranged. They were legal unions between families and often the couple never met before the ceremony. So important was the union of marriage that in ancient Greece and Rome unmarried men were denied access to certain public positions. The Greek lawgiver Solon contemplated making marriage mandatory and the Roman emperor Augustus actually passed laws mandating marriage and punishing those who did not marry.
The purpose of marriage in these cultures was not about an equal union or a union of partners. In Greek and much of Roman history, the man was dominant over his wife and family. The union was about producing off spring. So separated from our modern concept of marriage as a union of fidelity were the Greeks that the Greek orator Demosthenes explained: ""We have prostitutes for our pleasure, concubines for our health, and wives to bear us lawful offspring."
In ancient Israel, marriage too was radically different from our modern concept. Polygamy was permitted, Jacob married two sisters, Leah and Rachel. Again, arranged marriages were common. The father arranged the marriage for his son paying a "bride price" to the woman's father.
It is clear that we do not want the historical form of marriage. People should be free to choose their own partner. But historically, marriage was not just about the union of a man and a woman. It was about having children and raising a family. Clearly under that standard, gay marriage makes no sense.
Today, marriage has a very different meaning. People pair off voluntarily. Some put off marriage until later in life or do not marry at all. Couples may choose to have children together or may have no children. Marriage is a partnership. This modern concept of marriage is very much amicable to gay marriage. As our knowledge about human reproduction has increased, it is now even possible to allow for gay couples to raise children together without first having to conceive a child with the opposite sex. Perhaps future advances will allow the creation of offspring who's genetic parents are of the same gender.
In conclusion, marriage is not the same as it was historically. The original purpose of marriage, reproduction, is not the modern purpose. Gay marriage is the next step in our changing views of marriage. Even if you wish to impose the old tradition of having children and raising a family on marriage, science is allowing us to accomplish that end with same sex couples.