1. The reason for the criteria is that human judges cannot use the supernatural to decide the law. That which humans cannot observe, the law cannot dictate. And America is built upon a foundation of individual liberty. My personal, moral disapproval of someone else's actions is not enough reason to restrict their freedom to that action. Our government is hamstrung by the constitution. It is not allowed to restrict our freedom without sufficient justification. It is not allowed to classify its citizens differently without sufficient justification. Such justifications have to come from something identifiable. To restrict someone's freedom, you need to demonstrate why. I don't approve of Scientologists. Their beliefs are reprehensible. In my opinion, it's a cult that tricks people to get their money. But my disapproval is not enough to prevent Bob from joining them, or believing what they believe.
I want tighter gun control. But the government can't do that without a good enough reason. And because gun ownership is a right stipulated in the constitution, the bar for this is quite high. The need must be great, and the measure must be the least restrictive measure possible. Well, marriage is also a fundamental right. Marriage is a basic component of the family unit, which is the building block of a society. The right is so fundamental that we cannot even deny it to a criminal. Someone currently in jail still must be allowed to get married, and those are people who have undergone due process, those are people who have had the most basic rights imaginable stripped from them. Any restriction on who can marry who has to be met with a powerful justification.
While you might personally believe that this family unit must be based on a man and a woman, the government doesn't have the luxury of leaning on belief alone. If the legal contract exists, any private citizen must be allowed to enter into it barring a sufficient state interest in preventing it. Because the government is hamstrung by our constitution, and we made it that way to preserve individual freedom. Under the 14th amendment, a distinction of gender must be justified by an "important state interest" that the measure is "substantially related" to furthering. That's it. That's the test. Marriage contract, business contract, speed limits, doesn't matter. That's the test you need to pass for the state to sanction a distinction of gender.
2. As for your question, I believe that if it's not measurable, or empirical, "truth" will always be subjective to some degree. Your belief in God's will may be absolute, and that a particular reading of his word is absolute. Thing is, there's another guy who has a different reading of the same words, and he believes that to be absolute. A while back, people said similar things about Zeus. A mortal court system can't judge these things. Robbery causes specific, measurable harm to the victim. A wiccan casting a blood curse upon you does not. So the wiccan gets to chant their words over a bowl of newts, or whatever it is they do.