A scenario: a woman is married in the Catholic Church, abused by her spouse, and later seeks a civil divorce. She does not go through the church for an annulment. Later, she meets another man and is married to him by a Methodist minister.
Are they married? The Catholic Church, which does not recognize divorce and has not granted an annulment, would say, "No."
The State in which the Methodist minister performs the ceremony, however, says, "Yes." These two individuals are recognized by the State as legally joined in economic, social, and legal terms. They are collectively responsible for debts/taxes they incur. They own property together. They qualify automatically as next-of-kin in health-related situations. They are co-guardians of any children they have or adopt. They qualify for spousal benefits on health insurance.
The Catholic Church still says they aren't married--and in a religious sense, they may not be. But what they do have is a recognized civil union that the government and not the church can bestow and regulate.
Here's the truth that is so unpalatable for many American citizens: the government does not perform marriages; it only sanctions civil unions. Civil unions performed by clergy (or even justices of the peace) are usually called "marriages," but the irony is that it matters not whether a Priest, a Rabbi, or a Wiccan officiates as long as he/she possesses a government license to join people into civil unions--the advantages and disadvantages are the same.
Marriage is a traditional religious, cultural, and social term that governments have hijacked over the years to give a better smell to the civil unions it can create. All marriages in the USA are, in a legal sense, only civil unions.
So let's just get the government out of the marriage business, and admit that it only does civil unions.
And then let's get the hell out of the way of any two consenting adults who want to enter a civil union, whether they call it marriage or not.
Or any three, four, or seven consenting adults for that matter. I'm not precisely sure how your family arrangements--who sleeps with who, who does the dishes, and who wears what kind of underwear--are any of my business as long as you don't abuse your kids and don't mow your lawn before I get up on the weekends.
I would prefer a Constitutional Amendment to get this done; something like the following:
1. All legal references to "marriage" in law or policy shall henceforth and retroactively be considered equivalent to "civil union." The characterization of a civil union as a marriage shall be at the discretion of the parties involved.
2. No consenting adult shall be denied the right to enter a civil union with any other consenting adult on the basis of gender or sexual orientation.
Yeah, I know it's not that simple.
But it should be.