separation of church and state

I feel honored to say that my paternal (and possibly maternal...hmmmm....) family descends from Roger Williams. My favorite part about him is that he was very set in his religious views and yet he stood out as a light of tolerance and a provided a safe haven to people of all different religious views in a time of widespread intolerance. How wonderful is that? Here is my quick bio of this great man:

Born in the early 1600's in London. He was extremely smart and was awarded a scholarship to Pembroke College at Cambridge University. He later became a chaplain but came to the colonies (just 10 years after the mayflower) to escape the controversy surrounding him and his ideas on freedom of worship. He encountered much of the same resistance to his ideas in America though. One thing idea that he ended up being tried for was that he believed they needed to buy the land from it's rightful owners (the Native Americans) before it was an official deed. Before he was banished back to England he fled southwest out of the Massachusetts Bay area. He befriended the native Americans, learned their language and eventually set up a trading post with them. He bought land from them and founded Providence Rhode Island. He and his companions founded their new settlement upon the basis of complete religious toleration and it soon become a refuge and safe haven for all whom were prosecuted by the puritans. Although he was headstrong and rigid in his theological views, he was simultaneously extremely tolerant. One of his many contributions was his conviction that church and state should be separate. I like this quote from the Quartz Hill School of Theology: "Willaims wanted Church and State separated so the Church would not be corrupted by the State. Thomas Jefferson entertained the opposite conviction, fearing that the State would become contaminated by the Church. (Alpheus Thomas Mason. Free Government in the Making: Readings in American Political Thought. New York: Oxford University Press, 1965, p. 55)"

So I have just one more political rambling, and it concerns tolerance... and it is this: California just passed Prop 8, which would amend the state Constitution to restrict the definition of marriage to a union between a man and a woman. Regardless of whether you believe homosexuality is perfectly normal or morally wrong, I am wondering something. Why is it that marriage is tied up in both the state and the church? I can't help but wonder if both Williams and Jefferson's fears are coming true with this co-mingling of the church and state. Why don't we have civil unions for any couple wishing to make a commitment and get all the legal rights that come with marriage, and then separately anyone wishing too could also get married with the church and take the spiritual vows of marriage? That way the church would be responsible for determining it's definition of what "marriage" is, and then the state could be left out of that sticky mess and grant basic legal rights to people wishing to legally join together. I just don't understand why this isn't talked about more as a viable option. Any thoughts?