Marriage is an inherently religious idea which has become entangled with civil law in virtually every country in the world. Here’s my take on it for what it’s worth.
In the United States, everyone is guaranteed “equal protection under the law” by our constitution. To me that means that everyone should be entitled to the same rights and privileges of a domestic union, regardless of what it’s called. If “domestic partnerships” are authorized in lieu of “marriage” they must confer the exact same rights and privileges granted to married partners; they should in fact be the identical legal construct as a marriage. This is just basic human rights and should be unquestioned.
With that being said, no religious body should be required by law to perform a ceremony of marriage nor should they be penalized for declining to do so. That is basic religious freedom and should be guarded regardless of whether we agree with their beliefs or not.
I would go a step further and completely sever the religious ceremony of marriage from the legal status inferred by it. People should be free to form and dissolve domestic unions regardless of the approval or lack of from any religious body. The state should define what marriage is and the legal and civil process for forming it without respect to the teaching and practices of any established religion. People obviously should be free to seek the blessing of their faith in forming a marriage but that blessing should have no bearing on how the state views it.