what gives the state the right to define and delimit marriage is that the state is an organ of the society which it represents, defends and promotes. It is not some uber entity with powers and duties unconnected to the people of a particular place and time. The agents of the state can be few or many, but the nature of man (understood here in the universal sense) demands leadership and specialization.Though I left two long comments myself, I couldn't have said it better!
Whereas it is true that marriage has had and continues to be lived out in many different ways, yet three things at least persist: it is the union between male and female, it is the font from which new generations spring and are acculturated into society, and it is hedged about with rules as to who may marry whom, under what circumstances, when and how. Along with strong positive rules, it is also in like manner walled about with taboos.
Your easy dismissal of the past as mere *history* which can be dismissed in this enlightened age is pathetically naive. It partakes of the quintessential leftist notion that man is a construct molded by forces of which we were ignorant in former ages but which we now, in our wisdom, can understand, grasp and force to our wills, with no adverse consequences. I say naive because the events of the last century have certainly shown us the terrible effects of man seizing his destiny in his own hands and hammering his fellow man into the image of a new humanity by dint of fire, blood and a pure, transcendent and catastrophic ideology. Human nature will not be expunged by fiat. Not to say that human nature is immutable, but it is intrinsically conservative, containing within itself an inchoate wisdom distilled from the struggle of survival. The homosexual marriage movement is another in a series of moves which tend to erode and attenuate Western civilization. In thoughtlessly meddling with primordial anthropological mechanisms these anti-social fads destabilize the carefully balanced inner workings of civilization, and will inevitably produce the same kinds of cultural disasters with which recent history is littered.
That is why it is in society's best interest to strengthen rather than weaken marriage. Extending cultural approbation to homosexual marriage will have exactly that effect, a weakened marriage. And it will not stop there. Others in this discussion have pointed out that soon it will extend to multiple partners. Soon after that (you watch!) it will include non-humans. At that point it will have lost all meaning. Everything will have lost all meaning, for the very things that define us as persons, societies, civilizations, will have lost all meaning. We will have ceased to be human, devolved beyond all recognition into a masa damnata infesting the earth.
My original arguments are here, and recent slippery-slope events of great relevance are here.