The proposition that marriage should be a religious institution with no government involvement is a principled one but, aside from the fact that there are legitimate interests in marriage from those who are not religious, it is really irrelevant to the public ploicy debate of revising the definition of marriage.
Marriage is inextricably wound into the fabric and laws of this country, both state and federal, and is not going away. So, while I understand your position and might even have sympathy with it, it doesn't obtain.
The question before us is whether or not the definition of marriage should be expanded and if it is expanded, why should it be limited to a small subgroup of Americana.
From what I understand about the rule of law, we're faced with 2 choices. Either we convince the proponents of same sex marriage to leave things alone or we accomodate civil unions with legal associations and benefits similar to those afforded married couples but for couples - and groups of people - other than one man and one woman.
Frankly, I don't see either one being achieved easily. I wish that the pro-marriage factions would preempt the issue and create family corporations or communal associations under civil law --- and take marriage back to the church.
One way I make decisions about which laws I like and which I don't is to consider whether I am willing for that law to be enforced at the point of a gun (and whether I am willing to risk my own, a policeman's or soldier's life or liberty to enforce it).