If one starts a discussion about the definition of marriage, then one should be prepared for a long discussion. Recently this has made the news through the Supreme Court with a general rejection of the definition in the Defense of Marriage Act.
Now the union described by marriage can be used in various contexts. Unfortunately, if one takes popular usage as a guide, then marriage is primarily, and unavoidably, a religious term. A simple example is Catholicism where it is a sacrament of the Church. Therefore, any attempt to legally define marriage is inherently going to rub up against freedom of religion.
The simplest way to solve this would be to clearly separate legal and religious activities, preferably with a different term. I don’t want to be too cynical, but much of the debate seems to center around money, or property, or other wealth issues that we’ve tied to the state of marriage in our laws. If we take marriage out of our laws (for everyone), then we can relegate marriage to a purely religious ceremony.
Oddly enough, the defeats in the Supreme Court have made the proponents of defining marriage as purely a heterosexual union work on removing marriage from the legal framework. I don’t know if they realize it, but this is what they should have done to begin with. It reminds me of the quote from Churchill: “We can always count on the Americans to do the right thing, after they have exhausted all the other possibilities.”