Sorry, but I feel a need to ejaculate a bit of biological reality into this conversation.
At the time when the Constitution was written, it was commonly believed among the majority of the population that so-called "Negroes" were biologically inferior to "whites," and were simply incapable of functioning fully as citizens. Quotes from the Founding Fathers verifying this belief are not often published but are in the record. This is the reason why these enlightened people saw no contradiction in writing about the human rights of life, liberty, and so forth, while tacitly accepting the institution of slavery. Thus, the different treatment of "Blacks" was both constitutional and legal until the ratification of the 13th, 14th, and 15th Amendments, which gave the so-called "Negroes" the full rights of citizenship in the United States.
When those amendments were expanded to protect U.S. citizens as citizens of their respective states, the anti-miscegenation laws (sp?) were doomed.
But unlike the supposed differences between the races, the differences between men and women are not imaginary or supported merely by prejudices or illusions; they are real and verifiable either with or without scientific instrumentation. And "equal protection" has never been applied to "couples," but only to citizens. Furthermore, neither the Constitution nor the USSC has ever ruled that sexual preference is one of the categories - like gender, race, religion, etc - for which states must show a "compelling state interest" to treat one group differently from anyone else.
This is clearly an area where the States are free to discriminate (not all discrimination is bad), based on nothing more than the discretion of the population, as manifested by the peoples' legislatures (the state legislatures). And if the people, acting through legislation or ballot initiatives or whatever it may be, decide that marriage should be defined as ONLY a man and a woman who enter into a state-sanctioned quasi-contract, then that is the prevailing law.
And even with the hundreds of liberal judges and courts all across the fruited plain, who are DETERMINED to change the Constitution without benefit of a Constitutional Amendment, the state of (U.S.) "Constitutional" law on this point has not changed.
We do have many related issues that ar still "in play." It may not be possible for the Federal government to refuse to recognize "marriages" that the various states recognize. It may be necessary for all of the other states to recognize marriages legally created in another state, even if that marriage could not have come into existence in the latter state.
But as of this minute, the States, acting in their sovereign capacity, may REFUSE to accede to the demands of the loud-mouthed socialist, amoral pree-vert bastards who are trying to shove gay "marriage" down their throats (no play on words intended), and there is NOTHING in the United States Constitution that can even remotely be used to force the USSC to overturn the marriage laws of sovereign states who choose the traditional path in this area.
In short, the "equal protection" argument is bullshit.