Well, I have been happily married for quite a while now, so I think that I have some clue.
"Short answer: It's a *legal* definition of a sacred institution protecting real *families*."
The legality of marriage is not what makes marriage "sacred", nor is the legality what makes "a real family" sacred. The sanctity of marriage and family requires more than a simple legal document to render it so. The legality of marriage just makes it "legal", period -- and this is not irrelevant, but it isn't everything either. In fact, the legality of marriage is rarely even an issue to a member of a couple, until or unless there is a divorce or death of a spouse. Beyond that, it is the active participation in the emotional and spiritual responsibilities of marriage that has the most day-to-day relevance to a couple succeeding as and remaining "a family".
"Think of it as the legal underpinnings of western civilization and you wouldnt be too far off."
Actually, I would agree that "marriage" is one of the most important "underpinnings of Western Civilization" -- but that has been eroding for quite a long time now, independent of it definition as a legal institution.
What I am suggesting is more a "render unto Caesar what is Caesar's". We elect politicians to produces laws, not render things "sacred" or not. Yes, there needs to be a legal container for a personal partnership, but it is simply that -- a legality. Leave its sanctity to those involved and not to politicians.
Otherwise, you may get what you didn't bargain for -- rendering the concept of marriage to include homosexual couples and who knows what else, defiling its true sanctity and further eroding what has indeed been a fundamental pillar of Western Civilization.
"What I am suggesting is more a "render unto Caesar what is Caesar's". "
Marriage has been defined as a legal institution since the time of Caesar, in Western civilization as between one man and one woman. We can at least render some thanks that the Justinian Law of 500AD and every law since then defined marriage clearly in the West as one man and one woman.
The sanctity of marriage and family says something about its inherent goodness, but getting politicians involved is not about sanctifying anything, but about according it *legal protection*, which it needs to thrive. This is no different from saying that since religion is important, that we need a "freedom to worship" and tax exemption for churchs.
Only a fool would insist that a legal tax exemption 'sanctifies' a church!
It is sophistry, used often by those intending to weaken marriage's protections, to suggest that since something other than legality 'sanctifies' marriage, that therefore the legal underpinnings of marriage are unimportant.
That is a very wrong, very un-conservative or even anti-conservative argument to make, that thoroughly miscomprehends the role of law in this case. It's not an imposition of the law, its a matter of protection of something already understood as sacred in our culture. As we all know, legal structures undergird our public morality and incentivize our behavior. Read up on Edmund Burke and his 'little platoons' and the concept of Government as covenant between different generations.
Or in sociological behaviorist terms: Make marriage less palatable and less protected, and non-marriage thrives; allow polygamy/gay-unions as marriage, and culture decays; make divorce easier, and families get broken more. We see it already, as 'partners' get goodies once reserved for spouses, where the cultural inhibitions on sex, child-rearing, even family formation outside of marriage have fallen. etc. The negative consequences of the breakdown of family are too obvious and too legion not to notice.
"We elect politicians to produces laws, not render things "sacred" or not." Correct, and the most important laws they can pass or not pass are those that protect or not protect the vessels of our civilization. The people making the laws are like the curators at a great museum, and we entrust our important institutions to their temporary care to make sure the valuables of civilization not get stolen or damaged.
A definition of marriage in our laws is like repainting the Sistine Chapel ... in tangerine color.