All 50 states recognize civil unions or other relationship arrangements. At that point the courts just have to demand that states treat them the same as any other marriage. Not impossible, certainly not any more so than the seismic shift of redefining a centuries old definition of marriage.
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.