"... we've been sliding down it for 126 years and counting." I find nothing in that observation with which to disagree, but I'd apply it in a much broader context than marriage.
Please, do not misunderstand, I appreciate and value your comments, but am somewhat unconvinced that social or religious constructs really deserve to be considered "fundamental". Particularly ones with so apparently malleable a definition as marriage.
Clearly, in the context of United States case law, you make an unassailable point.