Contracts between you and me do not grant me legal rights from the government

Shouldn't they? That to me seems to be the crux of the whole argument.

If I want to sign a contract that makes someone not related to me by blood my "next of kin" shouldn't I be allowed to do so, and shouldn't society honor that contract? Likewise if the Government want to reward people who live in nuclear family units with lower taxes that's their perogative. I just don't see what "Marriage" as a social or religious construct has to do with it.