Currently in most countries, there are two things called marriage: One is a legally recognized agreement about property rights, child custody, taxes, and health benefits. The other is a relationship, recognized by the church, showing the union of a man and a woman before God.
The fact that these two very separate things are called marriage is a problem. The government should stay out of each church’s definition of marriage. The churches should stay out of the government’s ability to grant rights.
To clear things up, one of two things needs to happen: Either we need to make this distinction clear in law, or (even better) the word "marriage" needs to be completely removed from all laws, and replaced with "civil union" so that it will be clear that any two adults can be granted rights of that union, but it will also be clear the law does not say anything about what a (religious) marriage is.
One may then get married at whatever church one wants, with whichever definition of marriage that church recognizes—protected from any input by the government. This also protects each church from being forced to use any other religion's definition of marriage.
Property rights, child custody, taxes, and health benefits would then be granted to any two (or possibly more, depending on each contract) consenting adults, regardless of their relationship.
Oh, it looks like others have already thought of this: http://www.google.com/search?q="two+things+called+marriage"
These are some people who expressed the same thing probably better than expressed above:
Someone took a screen shot of the following post on a social networking site and shared it online: