Traditionally it has been one man and one woman for the marriage to produce an offspring. That is why a blood test is required. Somehow that has changed as some people marry and do not want to have children. The tax form and tax laws have been for one man and one woman as well. Again not a religious definition. If approved, same-sex marriage will redefine what marriage is and offend a lot of male-female marriages. It also might open the door to having multiple spouses, or marrying a non-human, or marrying oneself. So before a decision is made on same-sex marriages, the government is going to have to think about how it effects existing marriages and the loopholes it may cause.
It's interesting that you believe in the traditions of the judicial system and historical interpretation as it pertains to common law marriage, but eschew a similar vain of tradition which has arisen over the separation of Church and State. Indeed, one could easily argue that the question of what is or is not marriage is much more vague than the question of what constitutes establishment of religion by the state. In the olden days, if you spent an afternoon alone, you might well be considered married, regardless of whether you had consensual sex. Indeed, the idea of state sanctioned marriage is much more recent than the idea of marriage itself.
Common Law Marriage is much easier to understand than the seperation of church and state for me. Good thing only certain states allow Common Law Marriage, or one can find themselves married when they did not mean to get married.
As for expression of religion, you confuse the rights of the individual to express themselves religiously, with the right of that some person to express themselves as an officer of the state. I, for one, don't care to be stopped by a police officer and have that officer hand me a flyer about the upcoming revival meeting. Now, if that same officer wants to stand in front of the mall and hand out flyers, then it's fine by me, as long as said officer is in civilian clothing.
I would just smile and say "Thanks, but no thanks." myself. The same thing I say when they stop me and ask me if I want to contribute to the Police Officer's Ball or tickets to some Police event. If you consider it an abuse of power to stop someone just to give them a flyer, then I see your point. If that person was speeding and got pulled over, and then was offered a flyer, I see it as a different thing. Nobody is forcing them to take the flyer and go to the revival, if they are being forced then I see it as a bad thing.