I've no particular opinion on the state's definition of marriage, as my viewpoint is a tad more restrictive - i.e. marriage is a sacrament that's intended for life. But i'd rather the state not enforce that viewpoint.
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.
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.