As I have before, I would like to use this diary entry to test my thinking. My opinion is not set in stone: your reasoned responses might well help it evolve.
Marriage is a unique institution because it has a dual nature: it is something that can be bestowed by churches, synagogues and mosques, and it has a spiritual dimension; but it is also regulated by the state, and comes with rights and obligations that are of a legal and civic nature. Think of religion and government as mother and father to marriage.
The major religions are uniformly against the marriage of people of the same sex. I cannot agree with this opposition, but it is a fact. I think that to take the term 'marriage' and change its ground rules would be government imposing on the traditions and beliefs of a great many people. It would be like the father taking away the child from the mother.
At the same time, there is a compelling state interest in providing equal rights to its citizens. The federal government should not only allow states to offer a civil union alternative, they should mandate that the states do so under the equal protection provisions of the constitution. Furthermore, civil unions should offer every right and obligation that marriage does.
There are some on the religious right that say that gay marriage would de-sanctify the institution, but then go on to say that civil unions should not be allowed because they are basically the same thing as marriage. Listen to this argument: they are saying that the legal structure is the marriage... they are themselves debasing the spiritual content they claim to be defending. This is like the mother taking the child from the father.
It might surprise you to learn that I am an atheist, but I have experienced spiritual stirrings in my life, and I believe it is important to allow people to maintain their beliefs and practices.
You can say that I am not respecting the right of gay people to claim the mantle of spiritual union, but I think gays can bring spiritual meaning to their relationships without the imprimatur that one day, with broader religious and societal consensus, will surely become available to them.