In our country it's mostly the state governments. They issue marriage licenses and give certain privileges to married couples. The federal government treats married couples differently for income tax purposes. The following quotation is from Wikipedia:
"According to the United States Government Accountability Office (GAO), there are 1,138 statutory provisions in which marital status is a factor in determining benefits, rights, and privileges. These rights and responsibilities apply to only male-female couples, from the 1996 Defense of Marriage Act (DOMA), defining marriage as between a man and a woman."
I'm nowhere near the first to propose this, but I think marriage is basically a religious rite, and should be left entirely to the churches. (and of course synagogues, temples, etc.) The governments of the fifty states should deal with civil unions, AKA domestic partnerships.
This approach would end the current controversy over whether same sex marriages should be allowed. Some churches would perform same sex marriage and some would not. There would be controversies within many churches, but that's just a matter for those believers. Most states would recognize civil unions between two people; I think eventually all states would.
A few, very few, public figures have suggested that marriage, being traditionally a sacrament, should be something for religious organizations to deal with. Governments, on the other hand, should be responsible for defining and regulating civil unions, or domestic partnerships, however they might be named.
This makes a lot of sense to me. It would end all of the quarrelsome controversy over same sex marriage, except within individual churches. Each religion could make its own rules about marriage. Same sex couples presumably would qualify for civil unions. Those who also want to marry would have to find a church that accepts that. Disputes on that issue would take place within individual churches.
Each state government would have the responsibility of defining civil union and deciding how it works. Presumably, the vast majority would not specify that the partners need to be of opposite sexes. I think that eventually, all states would permit same sex domestic partnerships.
This also make it possible for the civil union concept to be extended toward non-sexual partnerships. There are many cases of long time friends, who are not lovers, and who have no other highly important relationships in their lives, who might benefit from a civil union, if it was divorced from sexuality. Civil unions might not even be restricted to pairs; small groups of single people who are very close friends might benefit from such an arrangement, and I see no downside for society.
So, in conclusion, I advocate that marriage be exclusively the domain of religion, and that government promote only domestic partnerships. Many couples, especially those with religion, would choose to have both. Although this is primarily an issue for the individual states, the federal government would have to confirm that the benefits, rights, and privileges formerly given to married couples, would now apply to domestic partnerships.