There is difference between a legal marriage and a religious marriage.
A legal marriage is a legal contract between two people. To achieve this, you must go through the court system, which from my knowledge has nothing to do with a church/religion. Legal marriage plays no part in religion.
The issue I guess is people think gay marriage will force churches to allow gay marriage. No it will not! A church can refuse to marry anyone because they can do what they want. But their wishes should not dictate US policy.
Being gay is no more of a choice than having brown eyes. Both are biological. The bible is a collection of fairy-tales written by men who did not know how the world worked. It is used to control people
Gay individuals just want equal rights under the law. They are not trying to change religious dogma.
You'll NEVER hear a secular reason against gay marriage. Once the government used to word 'marriage', it became secular. Seperation of church and state, people don't get it