Sorry, but your attempt to claim "separate but equal" fails to fit reality. The Full Faith and Credit Clause did not afford civil unions the same protections as marriages.
https://www.americanbar.org/groups/crsj/resources/human-righ...
"But suppose that you and your same-sex partner drive from Maryland to Vermont to enter into a civil union because it is the only state that recognizes a significant legal relationship between same-sex couples. If an accident occurs on your drive home and one of you is hospitalized, most states and institutions will not acknowledge your legal relationship with your spouse."
> seismic shift of redefining a centuries old definition of marriage.
Just as before, you are free to get a religious "marriage" that is unrecognized by the state. Just as before, the state can issue a marriage license to people that a particular religion does not approve (e.g., Orthodox rabbis are forbidden from officiating or validating non-Jewish weddings and do not recognize the resulting marriages, and the LDS church taught that interracial marriage, aside from between white men with native American women, was a sin until 2013). The only thing that changes is what marriages the state allows. There was no "seismic shift" outside the small minds of bigots.