As for the government staying out of the so called marriage business, that would be hard to do. Taxes, benefits, inheritances, obligations of parents, social security, and pension issues all revolve around marriage and the fact the govt. recognized that a married couple was a union (or a corporation of sorts).
The government by law has defined marriage in order to foster an orderly society.
All of which can be satisfied with civil unions.
Not any two people living in the same dwelling can say they are married. You have to make it official (legal) and there are different ways to do that. You don't need a church or a religious institution to make it happen.
Exactly, which is another reason for civil unions.
Religious opposition to calling gay unions gay marriages is there because many religious people do not consider what they have (a marriage between a man and a woman) the same as two people of the same sex, and for that reason they get upset as if the government and other forces are taking something special from them or that society is somehow diluting what they have(had).
If you consider how government and the agnostics in our country have slowly but surely changed things to fit their idea of how society should be; you can understand why people who believe marriage is something special and not just two people living together. Look how they took prayer out of schools, made abortion on the law of the land, made religious symbols offensive, and now this and anyone can understand why people are upset.
Again which suggests based on separation of church and state, government should not be involved with marriages since roughly 80% of marriages are associated with religion.
But like some here have posted, what have "we" lost? Really nothing, it's just that changes like this will certainly lead to other changes which will cost us something somehow. They always do.
Government sanctioning same sex marriage creates a conflict between freedom of religion and civil rights since religious institutions will not perform these marriages which no doubt will challenged by gay rights groups.