You and I may know it’s fundamental, as God says so in the Bible ... but I don’t believe it exists in the law, as in the U.S. Constitution.
HOWEVER, the people of this country can put it in the U.S. Constitution by the right they have of inserting a Constitutional Amendment. I would advise that route in order to solve all the wrangling over the issue.
Your argument seems to be that if the constitution doesn’t prohibit queer marriage, then states are required to allow it. I don’t see it that way at all.