This is something that was brought to mind by a current thread about gay marriage; I considered just posting this under that thread, but I wasn’t sure if this was completely within its scope and I didn’t wanna mess up someone else’s thread. 
That said, begin rant:
Pretty much everybody is probably familiar with the debates about gay marriage, etc., etc. The arguments usually focus on “gays have the right to marry too” or “homosexuality is wrong” or other such arguments. The problem is, these generally tend to be moral or ethical debates. But as far as I can tell, the purpose of the law is not to say what is right or what is wrong, but to enforce rules which create a stable and productive society. (Which is why theft, murder, etc. should be crimes; not because they are morally “wrong”, though they probably are, but because they would tend to foster an unstable society)
That said, take the entire concept of marriage. As far as I can tell, there are two possible benefits to society as a whole: The first is to assist those who are raising the next generation, which is clearly a vital part of any healthy society. The second is to allow people to form cohesive societal units which are more efficient and productive than two individuals alone would be. Either of these two provides a concrete and potentially demonstratable benefit to society. However, neither really lines up with our current (and I believe flawed) legal definition of marriage.
Take the first–raising the next generation. Raising a child is certainly not for the faint of heart, so extending special benefits to those who accept that responsibility is not unreasonable. But if you want to argue from this, then any number of hetereosexual couples don’t qualify. If the legal, societal purpose of the concept of “marriage” is to promote raising children, then all benefits should be extended to couples regardless of gender who are raising a child (biological or adopted) and withheld from anybody without children, again regardless of gender. (BTW, implied in this perspective is removing any obstacles to gay couples adopting children; that’s another argument, but allow me to make that assumption here.) Furthermore, one could even extend this argument to more than just groups of two people who are raising a child… but that’s details, and when one makes grand, sweeping arguments like I’m attempting to, you can’t afford to deal in specifics if you want to keep at any reasonable length.
There there’s the second perspective–formation of stable societal units. It is arguable that if you have two people who live together, share costs and own property collectively, this societal unit is capable of being more productive than two individuals living seperately. “The whole is greater than the sum of the parts,” and such. Again, this is an arguable and potentially demonstratable benefit to society, therefore extending legal benefits to such units would be reasonable. But from this perspective, why limit it to merely “couples”? What about two housemates who just happen to be good friends and want to pool resources? Or, perhaps, siblings who want to do the same? There’s no real reason–under this perspective–to distinguish between those sort of arrangements and a romantic relationship. Furthermore, under this perspective benefits should only be given to a unit shown to be stable–for instance, have benefits only apply once the people have been living together for X length of time, a year or suchlike. But that’s neither here nor there…
Now, you might be inclined to look at my two proposals and say that neither of those is a “real marriage”. That may be true, but we then get into the definitions of the word marriage which I don’t see as being terribly relevant. Perhaps it would be best to eliminate the word marriage from the law entirely; leave it to priests and ministers to marry people, and let the law concern itself just with legal matters… that is, what benefits society as a whole.
You may also want to point out that my proposals are unreasonable. And you know, you’d probably be right. This is just how I see things, and what I think would be superior to the current arrangement. I don’t necessarily expect that making such drastic changes to the current system would be feasible; it’s the principle of the matter.
At any rate, the reason I posted this massive missive here is because I suspect that there are people here who would disagree with me. 
So–I’ve explained what I think should be done with the marriage issue; is there any soul here who wouldst be so generous, as to show me how I am wrong?